An Azure service that is used to manage and protect cryptographic keys and other secrets used by cloud apps and services.
Access to Key Vault can only be granted to identities that are in the same AAD tenant as the Key Vault, so you will not be able to do what you want.
An alternative is to create a service principal in the Key Vault tenant and provide the credentials of this to the web app, for it to then use these to login to Key Vault.