It is difficult to truly offer a recommendation without knowing a bit more about your use case. It sounds like you're looking to take advantage of the convenience of hosting your project on Vercel. They've published a guide on how to achieve real time communication using their platform which you might find helpful.
In many cases, you need to pair real-time communication with persistence. This is where Supabase - my recommendation comes in. It is similar to Firebase, but it uses PostgreSQL under the hood, which makes it easier to understand and it's open source, with a cloud-based offering that includes a generous free tier.
If your application has some shared state, like a messenger, live whiteboard or a multiplayer game, you will need some way to store that state. Using the same technology for storage and updates will simplify your project.