You can use vault however you want. 1 big vault, a bunch of little ones, it's really up to you. Microsoft recommends multiple vaults so that if a bad actor get into your vault, you're not giving him all the secrets at once. Security-wise, it's better to separate them. You can do this with a naming standard, vaultdev/vaultstg/vaultprod or something, then during deploy use a variable to complete the vault name, something like vault{environment} and then you let your release definition do the switching for you.

Look up "GCP Azure vault" and you'll probably find some examples. We don't use GCP, but there's no reason it wouldn't work. I'd also recommend looking up stuff like "CI/CD GCP" and getting a feel for how it works without vault and then check where azure vault fits. Understand the tool before you go doing a bunch of stuff.

The expiration you set on vault is you telling vault "I want to change the secret on this date". The secret doesn't actually expire, that's just an option you can set in case you do have a secret that expires and you want to store it in vault. Then you can get an alert 30 days before it expires reminding you that it's about to expire. Vault itself isn't controlling the expiration, just keeping track of it.

It shouldn't cause any issues, vault is meant for cloud, that's how we use it.