First, you need to set up your routes. Then, according to Vercel, you need to provide a Handler process.

That's where you use the method use.

Handler = Proc.new do |req, res|
  app.use(req, res)
 end

In the rewrite configuration on your vercel.json file, make sure to rewrite every path (/(.*)) into your ruby file.

For example:

{
  "version": 2,
  "cleanUrls": true,
  "builds": [{
    "src": "/api/index.rb",
    "use": "@vercel/ruby"
  }],
  "rewrites": [{
    "source": "/(.*)",
    "destination": "/api/index.rb"
  }]
}