fix missing _identifying_params() in _VertexAICommon (#8303)

Full set of params are missing from Vertex* LLMs when `dict()` method is
called.

```
>>> from langchain.chat_models.vertexai import ChatVertexAI
>>> from langchain.llms.vertexai import VertexAI
>>> chat_llm = ChatVertexAI()
l>>> llm = VertexAI()
>>> chat_llm.dict()
{'_type': 'vertexai'}
>>> llm.dict()
{'_type': 'vertexai'}
```

This PR just uses the same mechanism used elsewhere to expose the full
params.

Since `_identifying_params()` is on the `_VertexAICommon` class, it
should cover the chat and non-chat cases.

libs/langchain/langchain/llms/vertexai.py

@@ -122,6 +122,11 @@ class _VertexAICommon(BaseModel):
             return enforce_stop_tokens(text, stop)
         return text
 
+    @property
+    def _identifying_params(self) -> Dict[str, Any]:
+        """Get the identifying parameters."""
+        return {**{"model_name": self.model_name}, **self._default_params}
+
     @property
     def _llm_type(self) -> str:
         return "vertexai"