diff --git a/src/google/adk/sessions/in_memory_session_service.py b/src/google/adk/sessions/in_memory_session_service.py
index 282e8e5..b2a84ef 100644
--- a/src/google/adk/sessions/in_memory_session_service.py
+++ b/src/google/adk/sessions/in_memory_session_service.py
@@ -11,6 +11,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+from __future__ import annotations
 
 import copy
 import logging
@@ -223,6 +224,7 @@ class InMemorySessionService(BaseSessionService):
       sessions_without_events.append(copied_session)
     return ListSessionsResponse(sessions=sessions_without_events)
 
+  @override
   async def delete_session(
       self, *, app_name: str, user_id: str, session_id: str
   ) -> None:
@@ -247,7 +249,7 @@ class InMemorySessionService(BaseSessionService):
         )
         is None
     ):
-      return None
+      return
 
     self.sessions[app_name][user_id].pop(session_id)
 
@@ -261,11 +263,20 @@ class InMemorySessionService(BaseSessionService):
     app_name = session.app_name
     user_id = session.user_id
     session_id = session.id
+
+    def _warning(message: str) -> None:
+      logger.warning(
+          f'Failed to append event to session {session_id}: {message}'
+      )
+
     if app_name not in self.sessions:
+      _warning(f'app_name {app_name} not in sessions')
       return event
     if user_id not in self.sessions[app_name]:
+      _warning(f'user_id {user_id} not in sessions[app_name]')
       return event
     if session_id not in self.sessions[app_name][user_id]:
+      _warning(f'session_id {session_id} not in sessions[app_name][user_id]')
       return event
 
     if event.actions and event.actions.state_delta:
diff --git a/src/google/adk/sessions/vertex_ai_session_service.py b/src/google/adk/sessions/vertex_ai_session_service.py
index 2cff001..5d6bed2 100644
--- a/src/google/adk/sessions/vertex_ai_session_service.py
+++ b/src/google/adk/sessions/vertex_ai_session_service.py
@@ -16,8 +16,8 @@ from __future__ import annotations
 import asyncio
 import logging
 import re
-import time
 from typing import Any
+from typing import Dict
 from typing import Optional
 import urllib.parse
 
@@ -50,9 +50,6 @@ class VertexAiSessionService(BaseSessionService):
     self.project = project
     self.location = location
 
-    client = genai.Client(vertexai=True, project=project, location=location)
-    self.api_client = client._api_client
-
   @override
   async def create_session(
       self,
@@ -86,6 +83,7 @@ class VertexAiSessionService(BaseSessionService):
     operation_id = api_response['name'].split('/')[-1]
 
     max_retry_attempt = 5
+    lro_response = None
     while max_retry_attempt >= 0:
       lro_response = await api_client.async_request(
           http_method='GET',
@@ -99,6 +97,11 @@ class VertexAiSessionService(BaseSessionService):
       await asyncio.sleep(1)
       max_retry_attempt -= 1
 
+    if lro_response is None or not lro_response.get('done', None):
+      raise TimeoutError(
+          f'Timeout waiting for operation {operation_id} to complete.'
+      )
+
     # Get session resource
     get_session_api_response = await api_client.async_request(
         http_method='GET',
@@ -235,11 +238,15 @@ class VertexAiSessionService(BaseSessionService):
   ) -> None:
     reasoning_engine_id = _parse_reasoning_engine_id(app_name)
     api_client = _get_api_client(self.project, self.location)
-    await api_client.async_request(
-        http_method='DELETE',
-        path=f'reasoningEngines/{reasoning_engine_id}/sessions/{session_id}',
-        request_dict={},
-    )
+    try:
+      await api_client.async_request(
+          http_method='DELETE',
+          path=f'reasoningEngines/{reasoning_engine_id}/sessions/{session_id}',
+          request_dict={},
+      )
+    except Exception as e:
+      logger.error(f'Error deleting session {session_id}: {e}')
+      raise e
 
   @override
   async def append_event(self, session: Session, event: Event) -> Event:
@@ -266,7 +273,7 @@ def _get_api_client(project: str, location: str):
   return client._api_client
 
 
-def _convert_event_to_json(event: Event):
+def _convert_event_to_json(event: Event) -> Dict[str, Any]:
   metadata_json = {
       'partial': event.partial,
       'turn_complete': event.turn_complete,
@@ -318,7 +325,7 @@ def _convert_event_to_json(event: Event):
   return event_json
 
 
-def _from_api_event(api_event: dict) -> Event:
+def _from_api_event(api_event: Dict[str, Any]) -> Event:
   event_actions = EventActions()
   if api_event.get('actions', None):
     event_actions = EventActions(