diff --git a/litellm/llms/vertex_ai/vector_stores/search_api/transformation.py b/litellm/llms/vertex_ai/vector_stores/search_api/transformation.py
index 14a0a406dff..a6fbe33a866 100644
--- a/litellm/llms/vertex_ai/vector_stores/search_api/transformation.py
+++ b/litellm/llms/vertex_ai/vector_stores/search_api/transformation.py
@@ -133,22 +133,25 @@ class VertexSearchAPIVectorStoreConfig(BaseVectorStoreConfig, VertexBase):
         extra_body: Optional[Dict[str, Any]] = None,
     ) -> Tuple[str, Dict[str, Any]]:
         """
-        Transform search request for Vertex AI RAG API
+        Transform a search request for the Vertex AI Search (Discovery Engine) API.
+
+        Per-request params pass through to the engine: max_num_results maps to
+        pageSize, and extra_body is merged in raw and takes precedence, so callers
+        can send native Discovery Engine fields such as dataStoreSpecs, filter,
+        boostSpec, or contentSearchSpec.
         """
-        # Convert query to string if it's a list
         if isinstance(query, list):
             query = " ".join(query)
 
-        # Vertex AI RAG API endpoint for retrieving contexts
         url = f"{api_base}:search"
 
-        # Construct full rag corpus path
-        # Build the request body for Vertex AI Search API
-        request_body = {"query": query, "pageSize": 10}
+        request_body: Dict[str, Any] = {"query": query, "pageSize": 10}
+        max_num_results = vector_store_search_optional_params.get("max_num_results")
+        if max_num_results is not None:
+            request_body["pageSize"] = max_num_results
+        if isinstance(extra_body, dict):
+            request_body.update(extra_body)
 
-        #########################################################
-        # Update logging object with details of the request
-        #########################################################
         litellm_logging_obj.model_call_details["query"] = query
 
         return url, request_body
diff --git a/tests/test_litellm/llms/vertex_ai/test_vertex_ai_search_vector_store_transformation.py b/tests/test_litellm/llms/vertex_ai/test_vertex_ai_search_vector_store_transformation.py
index 5ca71dc08c3..83d9fd703d8 100644
--- a/tests/test_litellm/llms/vertex_ai/test_vertex_ai_search_vector_store_transformation.py
+++ b/tests/test_litellm/llms/vertex_ai/test_vertex_ai_search_vector_store_transformation.py
@@ -1,3 +1,5 @@
+from types import SimpleNamespace
+
 import pytest
 
 from litellm.llms.vertex_ai.vector_stores.search_api.transformation import (
@@ -126,3 +128,69 @@ def test_should_raise_when_neither_engine_id_nor_vector_store_id_provided():
                 "vertex_location": "global",
             },
         )
+
+
+_ENGINE_BASE = (
+    "https://discoveryengine.googleapis.com/v1/projects/p/locations/global/"
+    "collections/default_collection/engines/app-2/servingConfigs/default_serving_config"
+)
+
+
+def _search_request(**overrides):
+    kwargs = dict(
+        vector_store_id="vs",
+        query="hello",
+        vector_store_search_optional_params={},
+        api_base=_ENGINE_BASE,
+        litellm_logging_obj=SimpleNamespace(model_call_details={}),
+        litellm_params={},
+    )
+    kwargs.update(overrides)
+    return VertexSearchAPIVectorStoreConfig().transform_search_vector_store_request(
+        **kwargs
+    )
+
+
+def test_search_request_defaults_to_query_and_pagesize_10():
+    url, body = _search_request()
+
+    assert url == _ENGINE_BASE + ":search"
+    assert body == {"query": "hello", "pageSize": 10}
+
+
+def test_search_request_maps_max_num_results_to_pagesize():
+    _, body = _search_request(
+        vector_store_search_optional_params={"max_num_results": 25}
+    )
+
+    assert body["pageSize"] == 25
+
+
+def test_search_request_passes_datastorespecs_through_extra_body():
+    specs = [
+        {
+            "dataStore": "projects/p/locations/global/collections/default_collection/dataStores/ds-beta"
+        }
+    ]
+
+    _, body = _search_request(extra_body={"dataStoreSpecs": specs})
+
+    assert body["dataStoreSpecs"] == specs
+    assert body["query"] == "hello"
+    assert body["pageSize"] == 10
+
+
+def test_search_request_extra_body_takes_precedence_over_defaults():
+    _, body = _search_request(
+        vector_store_search_optional_params={"max_num_results": 5},
+        extra_body={"pageSize": 50, "filter": 'category: ANY("docs")'},
+    )
+
+    assert body["pageSize"] == 50
+    assert body["filter"] == 'category: ANY("docs")'
+
+
+def test_search_request_joins_list_query():
+    _, body = _search_request(query=["foo", "bar"])
+
+    assert body["query"] == "foo bar"