## Problem

The Vertex AI handler returns a deferred stream object before any bytes are read from the network. When the router's fallback logic catches an exception, it only sees the successful stream-object creation — the actual HTTP error (4xx/5xx) is hidden inside the generator and only surfaces when the caller begins iterating. Fallback never triggers for Vertex AI HTTP errors on the streaming path.

## Proposed fix

Eagerly fetch the first chunk of the Vertex AI stream immediately after creation so that any HTTP-level errors raise before the result is returned to the router. This makes Vertex AI streaming errors observable in the same location as sync errors, allowing fallback to function correctly.

## Files

- `litellm/llms/vertex_ai/` (streaming handler)