langchain/chat_models/vertexai.py/ChatVertexAI/_agenerate
class ChatVertexAI:    async def _agenerate(
        self,
        messages: List[BaseMessage],
        stop: Optional[List[str]] = None,
        run_manager: Optional[AsyncCallbackManagerForLLMRun] = None,
    ) -> ChatResult:
        raise NotImplementedError(
            """Vertex AI doesn't support async requests at the moment."""
        )