Hi Team,

I have two feature requests related to the Vertex AI endpoint in LiteLLM:

1. Per-user cost tracking for Vertex AI endpoint

Our organization has adopted LiteLLM as our LLM gateway to enable better cost attribution on a per-user basis. We use Google's ADK (Agent Development Kit), and we've found that when routing through OpenAI-compatible endpoints, certain ADK-specific attributes are dropped when targeting Gemini models. To avoid this, we'd prefer to use the Vertex AI endpoint directly. However, we noticed that the Vertex AI endpoint currently does not support per-user tracking. Could this be added?

2. Support for the Interactions API on the Vertex AI endpoint

Gemini has introduced the new Interactions API with generateContent now considered legacy. Could LiteLLM add support for the Interactions API when using the Vertex AI endpoint?

Thanks! <img width="753" height="393" alt="Image" src="https://github.com/user-attachments/assets/275782ef-b835-4e32-8c34-797cc249c0bb" />