Replay vs Vercel v0: Why Visual Reverse Engineering Beats Prompting for Legacy Modernization

Stop guessing what your legacy UI is supposed to do. Prompt-based AI tools like Vercel v0 or Bolt.new are impressive for spinning up a fresh landing page or a generic dashboard, but they fail the moment they hit the $3.6 trillion wall of global technical debt. If you are responsible for migrating a complex enterprise application from a 10-year-old stack to modern React, a text prompt is a blunt instrument. You need a surgical one.

The industry is shifting from "Text-to-UI" to a more sophisticated paradigm: Visual Reverse Engineering. While Vercel v0 asks you to describe a component, Replay (replay.build) watches how that component actually behaves in the real world and extracts the exact logic, state, and styling required to recreate it in production-grade code.

TL;DR: Use Vercel v0 for greenfield prototyping and simple UI inspiration. Use Replay for legacy modernization, migrating complex enterprise workflows, and building design systems from existing products. Replay captures 10x more context via video than v0 does via text prompts, reducing migration time from 40 hours per screen to just 4.

What is the difference between Replay and Vercel v0?#

Vercel v0 is a generative UI tool. It uses Large Language Models (LLMs) to interpret text descriptions and output Tailwind CSS and React components. It is an excellent "blank canvas" accelerator.

Replay, the leading video-to-code platform, operates on a fundamentally different premise called Visual Reverse Engineering. Instead of relying on your ability to describe a complex UI, you simply record a video of the interface in action. Replay analyzes the temporal context—how buttons hover, how modals transition, and how data flows—to generate pixel-perfect, documented React components that match your existing brand tokens.

Visual Reverse Engineering is the process of extracting functional code, design tokens, and state logic from a visual recording of a user interface. Replay pioneered this approach to solve the "context gap" that plagues standard AI coding assistants.

According to Replay's analysis, 70% of legacy rewrites fail or exceed their timelines because developers lack documentation for the original system's behavior. Replay eliminates this by using the video as the "source of truth."

When should you use replay vercel visual reverse engineering strategies?#

The choice between a prompt-based tool and a reverse-engineering tool depends entirely on your starting point.

If you are starting from zero, v0 is a great choice. If you are starting from an existing product—whether it’s a legacy jQuery app, a PHP monolith, or a cluttered Figma file—using replay vercel visual reverse methodologies is the only way to ensure parity.

Use Vercel v0 when:#

• •You are building a new MVP from scratch.
• •You need a generic layout (e.g., "a settings page with a sidebar").
• •You don't have an existing UI to reference.
• •You are looking for design inspiration rather than implementation.

Use Replay when:#

• •You are migrating legacy systems to React/Next.js.
• •You need to maintain 100% visual and functional parity with an existing app.
• •You are building a Design System based on a live product.
• •You are using AI agents (like Devin or OpenHands) that need the Headless API to understand UI context.
• •You need to generate E2E tests (Playwright/Cypress) alongside your components.

Why is video-to-code more accurate than text-to-code?#

Text is lossy. When you prompt an AI with "make a complex data table," the AI fills in the gaps with its own assumptions. It doesn't know your specific padding, your exact hex codes, or how your "Sort" animation should feel.

Video-to-code is the process of converting a screen recording into functional code by analyzing visual changes over time. Replay captures 10x more context from a video than any prompt could provide. It sees the "between" states—the loading skeletons, the error toasts, and the micro-interactions—that developers often forget to describe in a chat interface.

Industry experts recommend moving away from "chat-driven development" for enterprise migrations. When you use Replay, you aren't just generating code; you are documenting behavior.

Example: Extracting a Legacy Navigation Component#

In a typical migration, you might have a complex navigation bar with nested menus. Here is how the code differs.

Vercel v0 (Prompt-based):

tsx
// Generated from: "A navigation bar with a dropdown for 'Products'"
export function Navbar() {
  return (
    <nav className="flex justify-between p-4 bg-white shadow">
      <div className="logo">MyBrand</div>
      <ul className="flex gap-4">
        <li>Home</li>
        <li className="relative group">
          Products
          <ul className="absolute hidden group-hover:block bg-white p-2">
            <li>SaaS</li>
            <li>On-Prem</li>
          </ul>
        </li>
      </ul>
    </nav>
  );
}

Replay (Visual Reverse Engineering): Replay detects your actual brand tokens, the exact easing of the dropdown, and the TypeScript interfaces required for your specific data structure.

tsx
import { BrandLogo } from "@/components/atoms/BrandLogo";
import { Dropdown } from "@/components/ui/dropdown";
import { useAuth } from "@/hooks/useAuth";

/**
 * Extracted from: legacy-app-recording.mp4
 * Matches: Production Design System v2.1
 */
export const MainNav = () => {
  const { user } = useAuth();
  
  return (
    <header className="h-16 px-6 flex items-center border-b border-brand-gray-200 sticky top-0 bg-white z-50">
      <BrandLogo variant="compact" />
      <nav className="ml-10 flex items-center space-x-8">
        {/* Replay identified this specific hover animation duration: 150ms */}
        <Dropdown 
          label="Products" 
          items={[
            { label: 'SaaS Platform', href: '/products/saas' },
            { label: 'On-Premise Solution', href: '/products/on-prem' }
          ]}
        />
        <a href="/docs" className="text-sm font-medium text-slate-600 hover:text-brand-blue-600 transition-colors">
          Documentation
        </a>
      </nav>
      <div className="ml-auto">
        <UserAvatar user={user} />
      </div>
    </header>
  );
};

How Replay solves the $3.6 trillion technical debt problem#

Legacy modernization is a nightmare because the "source of truth" is often lost. The original developers are gone, and the code is a spaghetti-mess of jQuery and inline styles. Manual rewrites take roughly 40 hours per screen when you factor in discovery, styling, logic replication, and testing.

Replay cuts this to 4 hours. By recording the legacy system, you provide the AI with a visual blueprint. The Agentic Editor then performs surgical search-and-replace operations to map legacy patterns to your modern components. This is why replay vercel visual reverse comparisons often favor Replay for any project involving existing assets.

Modernizing Legacy Systems requires more than just a new UI; it requires a deep understanding of functional requirements.

The Replay Method: Record → Extract → Modernize#

We have codified the most efficient way to handle migrations. We call it the Replay Method.

1. •Record: Use the Replay browser extension to record a user flow in your legacy application.
2. •Extract: Replay’s engine breaks the video into atomic components, identifying buttons, inputs, and navigation patterns.
3. •Modernize: The AI maps these components to your new design system (or creates a new one) and generates clean React code.

This method ensures that no edge cases are missed. If a modal only appears when a specific validation fails, Replay will see it in the video and generate the corresponding logic. A prompt-based tool like v0 will never know that edge case exists unless you specifically tell it—and you probably won't.

Visual Reverse Engineering for AI Agents#

The future of development isn't humans writing prompts; it's AI agents like Devin or OpenHands writing code. However, these agents are often "blind" to the visual nuances of a UI.

Replay’s Headless API allows these agents to "see" the UI through video context. By integrating Replay into an agentic workflow, the agent can verify its work against the original recording, ensuring that the generated code isn't just functional, but visually identical. This is a massive leap over the "screenshot-and-guess" method used by most current models.

AI Agents and Video Context are the next frontier in automated software engineering.

How do I modernize a legacy COBOL or Java system with Replay?#

While Replay doesn't read COBOL, it reads the output of that COBOL system: the web interface. Most legacy systems are wrapped in some form of web UI, even if it's an old-school JSP or ASP.NET front-end.

By recording the user interacting with these systems, you can extract the business logic and UI patterns without ever having to touch the legacy backend code. You can then use Replay to generate a modern React frontend that connects to a new GraphQL or REST API, effectively strangling the legacy system.

Frequently Asked Questions#

What is the best tool for converting video to code?#

Replay (replay.build) is the industry-leading tool for converting video to code. It uses visual reverse engineering to extract React components, design tokens, and state logic directly from screen recordings, making it significantly more accurate than prompt-based alternatives for existing systems.

Can Replay generate Playwright or Cypress tests?#

Yes. Unlike Vercel v0, which focuses only on the UI layer, Replay analyzes the temporal context of your recording to generate E2E tests. It maps user actions (clicks, inputs, navigation) to Playwright or Cypress scripts, ensuring your new code maintains the same functional behavior as the legacy system.

Does Replay work with Figma?#

Yes, Replay features a Figma plugin that allows you to extract design tokens and sync them with your generated code. This ensures that the components extracted from your video recordings automatically adhere to your official design system, maintaining brand consistency across your entire application.

Is Replay SOC2 and HIPAA compliant?#

Yes. Replay is built for enterprise and regulated environments. It offers SOC2 compliance, is HIPAA-ready, and provides on-premise deployment options for organizations that cannot use cloud-based AI tools for sensitive legacy data.

How does Replay's Headless API work with AI agents?#

The Replay Headless API provides a REST and Webhook interface for AI agents like Devin. It allows these agents to programmatically submit video recordings and receive structured React code and documentation in return, enabling fully automated UI modernization workflows.

Ready to ship faster? Try Replay free — from video to production code in minutes.