diff --git a/litellm/llms/vertex_ai/vertex_llm_base.py b/litellm/llms/vertex_ai/vertex_llm_base.py
index 8b7f8c63625..ca6d6c680e4 100644
--- a/litellm/llms/vertex_ai/vertex_llm_base.py
+++ b/litellm/llms/vertex_ai/vertex_llm_base.py
@@ -127,27 +127,48 @@ class VertexBase:
     ) -> tuple[_VertexCredentialsObject | None, str]:
         if credentials is not None:
             if isinstance(credentials, str):
-                _is_path: Final = os.path.exists(
-                    credentials
-                )  # credentials is from server config (litellm_params), not user input
                 verbose_logger.debug(
-                    "Vertex: Loading vertex credentials, is_file_path=%s, current dir %s",
-                    _is_path,
+                    "Vertex: Loading vertex credentials, current dir %s",
                     os.getcwd(),
                 )
 
-                try:
-                    if _is_path:
+                # Decide by value shape, not os.path.exists(): exists()
+                # swallows every OSError (missing, unreadable, transient),
+                # which previously misreported all of them as invalid JSON.
+                if credentials.lstrip().startswith("{"):
+                    try:
+                        json_obj = json.loads(credentials)
+                    except json.JSONDecodeError as e:
+                        raise Exception(
+                            "Unable to load vertex credentials from environment. "
+                            "Ensure the JSON is valid (check for unescaped newlines in private_key). "
+                            f"Parse error: {type(e).__name__}: {e}"
+                        ) from e
+                else:
+                    try:
                         with open(credentials) as f:
                             json_obj = json.load(f)
-                    else:
-                        json_obj = json.loads(credentials)
-                except Exception as e:
-                    raise Exception(
-                        "Unable to load vertex credentials from environment. "
-                        "Ensure the JSON is valid (check for unescaped newlines in private_key). "
-                        f"Parse error: {type(e).__name__}"
-                    )
+                    except FileNotFoundError as e:
+                        raise Exception(
+                            f"Unable to load vertex credentials from file path: {credentials}. File not found. ({e})"
+                        ) from e
+                    except PermissionError as e:
+                        raise Exception(
+                            "Unable to load vertex credentials from file path: "
+                            f"{credentials}. File not readable (permission denied). ({e})"
+                        ) from e
+                    except OSError as e:
+                        raise Exception(
+                            "Unable to load vertex credentials from file path: "
+                            f"{credentials}. Unable to read file. ({e})"
+                        ) from e
+                    except (json.JSONDecodeError, UnicodeDecodeError) as e:
+                        raise Exception(
+                            "Unable to load vertex credentials from file path: "
+                            f"{credentials}. Ensure the JSON is valid "
+                            "(check for unescaped newlines in private_key). "
+                            f"Parse error: {type(e).__name__}: {e}"
+                        ) from e
             elif isinstance(credentials, dict):
                 json_obj = credentials
             else:
diff --git a/tests/test_litellm/llms/vertex_ai/test_vertex_llm_base.py b/tests/test_litellm/llms/vertex_ai/test_vertex_llm_base.py
index a4d67606698..3906df60de3 100644
--- a/tests/test_litellm/llms/vertex_ai/test_vertex_llm_base.py
+++ b/tests/test_litellm/llms/vertex_ai/test_vertex_llm_base.py
@@ -2193,3 +2193,36 @@ class TestVertexBase:
 
             assert token == "cached-token"
             assert not mock_get_lock.called, "Fast path should not acquire lock"
+
+
+class TestLoadAuthCredentialFileErrors:
+    def test_missing_credential_file_names_path(self):
+        vertex_base = VertexBase()
+        with pytest.raises(Exception, match="File not found"):
+            vertex_base.load_auth(
+                credentials="/nonexistent/vertexai.json", project_id="p"
+            )
+
+    def test_unreadable_credential_file_names_path(self):
+        vertex_base = VertexBase()
+        with patch(
+            "builtins.open", side_effect=PermissionError(13, "Permission denied")
+        ):
+            with pytest.raises(Exception, match="not readable"):
+                vertex_base.load_auth(
+                    credentials="/some/vertexai.json", project_id="p"
+                )
+
+    def test_malformed_credential_file_keeps_json_advice(self, tmp_path):
+        bad_file = tmp_path / "vertexai.json"
+        bad_file.write_text("{not json")
+        vertex_base = VertexBase()
+        with pytest.raises(Exception, match="Ensure the JSON is valid"):
+            vertex_base.load_auth(
+                credentials=str(bad_file), project_id="p"
+            )
+
+    def test_malformed_inline_json_keeps_environment_message(self):
+        vertex_base = VertexBase()
+        with pytest.raises(Exception, match="from environment"):
+            vertex_base.load_auth(credentials="{not json", project_id="p")