fix(llm_passthrough_endpoints.py): initialize vertex client within route

mitigate credential caching issue

litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py

@@ -448,6 +448,8 @@ async def vertex_proxy_route(
         get_vertex_project_id_from_url,
     )
 
+    vertex_llm_base = VertexBase()
+
     encoded_endpoint = httpx.URL(endpoint).path
     verbose_proxy_logger.debug("requested endpoint %s", endpoint)
     headers: dict = {}