diff --git a/litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py b/litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py
index c116960a1ff..ded975ac795 100644
--- a/litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py
+++ b/litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py
@@ -604,14 +604,13 @@ class VertexGeminiConfig(VertexAIBaseConfig, BaseConfig):
                 openai_function_object = _openai_function_object
 
             elif "name" in tool:  # functions list
-                _named_function_object = ChatCompletionToolParamFunctionChunk(**tool)
-
-                if _named_function_object.get("parameters") is None:
-                    _input_schema = tool.get("input_schema")
-                    if isinstance(_input_schema, dict):
-                        _named_function_object["parameters"] = _build_vertex_schema(_input_schema)
-
-                openai_function_object = _named_function_object
+                _input_schema = tool.get("input_schema") if tool.get("parameters") is None else None
+                _named_tool = (
+                    {**tool, "parameters": _build_vertex_schema(_input_schema)}
+                    if isinstance(_input_schema, dict)
+                    else tool
+                )
+                openai_function_object = ChatCompletionToolParamFunctionChunk(**_named_tool)
 
             if "type" in tool and tool["type"] == "computer_use":
                 computer_use_config = {k: v for k, v in tool.items() if k != "type"}
diff --git a/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py b/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py
index 55559b441ba..1ef46cdb86f 100644
--- a/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py
+++ b/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_and_google_ai_studio_gemini.py
@@ -1254,14 +1254,32 @@ def test_vertex_ai_map_tool_input_schema_gets_vertex_schema_conversion():
 
 def test_vertex_ai_map_tool_explicit_parameters_wins_over_input_schema():
     """A tool carrying both keys must keep `parameters` — `input_schema` is only a fallback."""
-    tool = _anthropic_shaped_tool({"type": "object", "properties": {"ignored": {"type": "string"}}})
-    tool["parameters"] = {"type": "object", "properties": {"location": {"type": "string"}}}
+    tool = {
+        **_anthropic_shaped_tool({"type": "object", "properties": {"ignored": {"type": "string"}}}),
+        "parameters": {"type": "object", "properties": {"location": {"type": "string"}}},
+    }
 
     declaration = _declaration_for(tool)
 
     assert declaration["parameters"]["properties"] == {"location": {"type": "string"}}
 
 
+def test_vertex_ai_map_tool_null_parameters_falls_back_to_input_schema():
+    """A JSON `"parameters": null` alongside `input_schema` must take the fallback, not
+    send a null schema upstream."""
+    tool = {
+        **_anthropic_shaped_tool({"type": "object", "properties": {"location": {"type": "string"}}}),
+        "parameters": None,
+    }
+
+    declaration = _declaration_for(tool)
+
+    assert declaration["parameters"] == {
+        "type": "object",
+        "properties": {"location": {"type": "string"}},
+    }
+
+
 def test_vertex_ai_map_tool_without_any_schema_is_unchanged():
     """A named tool with no schema at all must still map, without inventing parameters."""
     declaration = _declaration_for({"name": "ping", "description": "no args"})