Instead of doing everything from scratch, you could support bitwarden API on server side and reuse all the existing clients. Or at least look at how they do it before you start your own.

What to watch out for? Honestly, you're missing some information regarding encryption - you can't store the passwords hashed and salted (you need to recover them completely) and aes-256 does neither of those operations. So you have to at least do some extra research.