I know that HCP Vault can only (currently) be deployed to AWS but is it possible to use with infra on other cloud platforms already? Or do we need to wait until HCP Vault can be provisioned on the same cloud platform as the rest of our infra?
In theory you can definitely use it with resources on other platforms. The hardest part I can see is that you would need to be routing traffic from your cloud via the VPC peer in AWS. You could use a public address with Vault but there are risks with that approach.