diff --git a/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py b/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
index 679ae3daa78..c64f3a9db2a 100644
--- a/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
+++ b/litellm/proxy/pass_through_endpoints/llm_passthrough_endpoints.py
@@ -23,7 +23,7 @@ from typing import TYPE_CHECKING, Annotated, Final, Literal, Protocol, cast
 
 import httpx
 import openai
-from fastapi import APIRouter, Depends, HTTPException, Request, Response, WebSocket
+from fastapi import APIRouter, Depends, HTTPException, Request, Response, Security, WebSocket
 from fastapi.responses import StreamingResponse
 from pydantic import ConfigDict, TypeAdapter, ValidationError
 from starlette.websockets import WebSocketState
@@ -77,6 +77,8 @@ from litellm.proxy.auth.handle_jwt import JWTHandler
 from litellm.proxy.auth.route_checks import RouteChecks
 from litellm.proxy.auth.user_api_key_auth import (  # noqa: F401  # legacy module exports
     _get_bearer_token,  # pyright: ignore[reportPrivateUsage,reportUnusedImport]  # backwards-compatible package export
+    api_key_header,
+    custom_litellm_key_header,
     get_bearer_token,
     is_no_auth_dev_mode,
     user_api_key_auth,
@@ -3137,6 +3139,16 @@ async def vertex_discovery_proxy_route(
     )
 
 
+async def _authenticate_vertex_caller(
+    request: Request,
+    authorization: Annotated[str | None, Security(api_key_header)],
+    litellm_api_key: Annotated[str | None, Security(custom_litellm_key_header)],
+) -> UserAPIKeyAuth:
+    return await user_api_key_auth(
+        request=request, api_key=authorization or "", custom_litellm_key_header=litellm_api_key
+    )
+
+
 @router.api_route(
     "/vertex-ai/{endpoint:path}",
     methods=["GET", "POST", "PUT", "DELETE", "PATCH"],
@@ -3152,7 +3164,7 @@ async def vertex_proxy_route(
     endpoint: str,
     request: Request,
     fastapi_response: Response,
-    user_api_key_dict: UserAPIKeyAuth = Depends(user_api_key_auth),
+    user_api_key_dict: Annotated[UserAPIKeyAuth, Depends(_authenticate_vertex_caller)],
 ):
     """
     Call LiteLLM proxy via Vertex AI SDK.
diff --git a/tests/unit/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py b/tests/unit/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py
index 376b2f18d3b..54874b51478 100644
--- a/tests/unit/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py
+++ b/tests/unit/proxy/pass_through_endpoints/test_llm_pass_through_endpoints.py
@@ -8128,11 +8128,14 @@ class TestAzureSpeechRawBodyThroughRealAuth:
 
 
 class _CountingCustomAuth:
-    def __init__(self) -> None:
+    def __init__(self, only_accepts: str | None = None) -> None:
+        self.only_accepts: Final = only_accepts
         self.api_keys_seen: list[str] = []
 
     async def __call__(self, request: Request, api_key: str) -> UserAPIKeyAuth:
         self.api_keys_seen.append(api_key)
+        if self.only_accepts is not None and api_key != self.only_accepts:
+            raise ValueError("unknown caller")
         return UserAPIKeyAuth(user_id=f"custom-auth-caller-{len(self.api_keys_seen)}")
 
 
@@ -8274,6 +8277,23 @@ class TestVertexPassthroughAuthenticatesOncePerRequest:
         assert custom_auth.api_keys_seen == [self.VIRTUAL_KEY]
         assert not google.called
 
+    @pytest.mark.parametrize("header", ["x-goog-api-key", "x-api-key", "API-Key", "Ocp-Apim-Subscription-Key"])
+    def test_key_outside_litellm_headers_is_rejected_and_never_reaches_google_without_a_vertex_credential(
+        self, monkeypatch: pytest.MonkeyPatch, header: str
+    ) -> None:
+        custom_auth: Final = _CountingCustomAuth(only_accepts=self.VIRTUAL_KEY)
+        with respx.mock(assert_all_called=False) as upstream:
+            google: Final = upstream.route().mock(return_value=httpx.Response(200))
+
+            response, recorder = self._post(
+                monkeypatch, f"/vertex_ai/{self.GENERATE_CONTENT}", {header: self.VIRTUAL_KEY}, custom_auth, ()
+            )
+
+        assert response.status_code == 401, response.text
+        assert self.VIRTUAL_KEY not in custom_auth.api_keys_seen
+        assert recorder.served_user_ids == []
+        assert not google.called
+
 
 class TestTypeSafePassthroughRoute:
     @staticmethod