Key Vault resource is designed to be per application/service like noted here: https://learn.microsoft.com/en-us/azure/key-vault/general/best-practices#use-separate-key-vaults
In terms of your scenario, it depends if your webservice are atomic (separate database, deployed separately) or not. You can think about AKV as deployment unit, so issue with deployment, app/service would not impact other app/services.