Can we use existing VPC, subnet in aws for fresh metaflow deployment?

yes. its slightly annoying (in terms of keeping up to date with "official" metaflow deployment) - you justhave to adapt the terraform/cloudformation.

If you use terraform it should be easy, it leaves VPC creation completely to the terraform module user. Check out the "minimal" example we have in the terraform repo, it shows how to create VPC using a popular community module and then use that with metaflow module.