### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### The Feature

Vertex AI Multi-Region Endpoints (mREPs) generate an incorrect URL when using `api_base` with `vertex_ai/` models.

Google recently introduced for Vertex AI, which guarantee data residency within a jurisdiction. The mREP base URL format is `https://aiplatform.us.rep.googleapis.com` (note: no region prefix like `us-central1-`).

**Expected URL format** (per Google's docs): ``` https://aiplatform.us.rep.googleapis.com/v1/projects/{PROJECT}/locations/us/publishers/google/models/{MODEL}:generateContent ```

**Actual URL generated by LiteLLM:** ``` https://aiplatform.us.rep.googleapis.com:generateContent ```

### Relevant code

In `litellm/llms/vertex_ai/vertex_llm_base.py`, the `_check_custom_proxy` method handles custom `api_base` for Vertex AI. When `api_base` is set and `use_psc_endpoint_format` is `False` (the default), it falls through to:

```python url = "{}:{}".format(api_base, endpoint) ```

This strips the entire `/v1/projects/.../locations/.../publishers/google/models/...` path and just appends `:{endpoint}` to the base URL.

The `use_psc_e…