Proper way to handle multiple domains and subdomains in vercel/nextjs for SaaS?

I'm building a SaaS and need to offer my customers the ability to have their own domains/subdomains pointed to their personal pages. That can easily done on Vercel.
However I'm not sure how to handle it in NextJS, I'm thinking of creating a [slug].js file for domains/subdomains and then rewrite from next.config.
Is that the proper way to handle it or is there a better way? Has anyone done this?

Thanks!

Edit: forgot to add that I'm using getStaticProps and I need to generate a static page.