My first suggestion is never ever use AWS. Of course I have violated that rule since their S3 storage is free and hard to pass up. The issue is whether vultr would kick them off. Seems unlikely. Vultr or Linode or Digitial Ocean or others might if there was a DDOS attack that affected their other customers. But otherwise I don't think they would.
True, and one thing I always try to do is keep my stuff portable - no using AWS/GCP/Azure proprietary stuff (lambdas, etc), so I can jump to a new platform provider if needed (containers are a big help here).
S3 is one exception, as you say, because it’s so dang useful. The good thing there is there are plenty of S3-compatible solutions out there (Digital Ocean’s spaces, minio, etc). I’ve used both and they’re (mostly) a drop-in replacement.