Currently Vercel does not support Strapi backend deployment. I would recommend Strapi Cloud dev plan, or I also use Digital Ocean or Render. I also heard some great things about Railway.
I have a project running Strapi Cloud at their dev tier 29 bucks, you get email, database and file storage included.
Also, our goal, in the future is to offer a free tier of Strapi Cloud. I just don’t have the ETA.