Vertex is a GCP service, and it's completely in line with how any other GCP service account would work; there's multiple auth flows, some taking credentials from the environment, and others from a file. From zero-trust POV, you could make an argument that long-lived "API keys" are effectively an anti-pattern. You may like this position, or you may dislike it, but it works. These are the guys that created Zanzibar[1] after all.

Also, AWS Bedrock and Azure AI Studio are similarly set up; the big cloud providers are big on security.

[1] https://research.google/pubs/zanzibar-googles-consistent-glo...