Vercel may not be the right place for strapi since it needs a node environment for its runtime… It does have to do some stuff like hitting endpoint, business logic in APIs, serving data to the admin UI / app-ui and writing data back to the db etc. etc.,
Supabase eliminates the db cost… you still need a web-server to host strapi… You can go for the AWS free tier which gives 12-month free micro ec2 instance (256mb linux) which would be sufficient for running a relatively simple low load strapi app / api