Initially opened the following discussion with vector upstream, to inquire on any plans of implementing GCP WIF authentication.

https://github.com/vectordotdev/vector/discussions/16278

Current issue is a generic error: "Invalid GCP Credentials", when using a type: "external_account" for the credentials file in vector.   It's likely this is a lack of the necessary logic in the current vector image.  I've taken a look at the upstream, and don't see a way for this to work as it stands.   I have inquired with vector upstream.

Now feature request has been created: 
https://github.com/vectordotdev/vector/issues/16387