diff --git a/litellm/llms/vertex_ai.py b/litellm/llms/vertex_ai.py
index d3bb2c78ab3..84fec734fd0 100644
--- a/litellm/llms/vertex_ai.py
+++ b/litellm/llms/vertex_ai.py
@@ -867,6 +867,8 @@ async def async_completion(
     Add support for acompletion calls for gemini-pro
     """
     try:
+        import proto  # type: ignore
+
         if mode == "vision":
             print_verbose("\nMaking VertexAI Gemini Pro/Vision Call")
             print_verbose(f"\nProcessing input messages = {messages}")
@@ -901,9 +903,21 @@ async def async_completion(
             ):
                 function_call = response.candidates[0].content.parts[0].function_call
                 args_dict = {}
-                for k, v in function_call.args.items():
-                    args_dict[k] = v
-                args_str = json.dumps(args_dict)
+
+                # Check if it's a RepeatedComposite instance
+                for key, val in function_call.args.items():
+                    if isinstance(
+                        val, proto.marshal.collections.repeated.RepeatedComposite
+                    ):
+                        # If so, convert to list
+                        args_dict[key] = [v for v in val]
+                    else:
+                        args_dict[key] = val
+
+                try:
+                    args_str = json.dumps(args_dict)
+                except Exception as e:
+                    raise VertexAIError(status_code=422, message=str(e))
                 message = litellm.Message(
                     content=None,
                     tool_calls=[
diff --git a/litellm/tests/test_amazing_vertex_completion.py b/litellm/tests/test_amazing_vertex_completion.py
index a56d7fe5a9a..ce9e6286ff2 100644
--- a/litellm/tests/test_amazing_vertex_completion.py
+++ b/litellm/tests/test_amazing_vertex_completion.py
@@ -590,19 +590,20 @@ def test_gemini_pro_vision_base64():
             pytest.fail(f"An exception occurred - {str(e)}")
 
 
+@pytest.mark.parametrize("sync_mode", [True, False])
 @pytest.mark.asyncio
-def test_gemini_pro_function_calling():
+async def test_gemini_pro_function_calling(sync_mode):
     try:
         load_vertex_ai_credentials()
-        response = litellm.completion(
-            model="vertex_ai/gemini-pro",
-            messages=[
+        data = {
+            "model": "vertex_ai/gemini-pro",
+            "messages": [
                 {
                     "role": "user",
                     "content": "Call the submit_cities function with San Francisco and New York",
                 }
             ],
-            tools=[
+            "tools": [
                 {
                     "type": "function",
                     "function": {
@@ -618,11 +619,13 @@ def test_gemini_pro_function_calling():
                     },
                 }
             ],
-        )
+        }
+        if sync_mode:
+            response = litellm.completion(**data)
+        else:
+            response = await litellm.acompletion(**data)
 
         print(f"response: {response}")
-    except litellm.APIError as e:
-        pass
     except litellm.RateLimitError as e:
         pass
     except Exception as e: