diff --git a/vertexai/preview/rag/utils/_gapic_utils.py b/vertexai/preview/rag/utils/_gapic_utils.py
index 4150111c60..6eae67fdfa 100644
--- a/vertexai/preview/rag/utils/_gapic_utils.py
+++ b/vertexai/preview/rag/utils/_gapic_utils.py
@@ -869,9 +869,7 @@ def set_vector_db(
 ) -> None:
     """Sets the vector db configuration for the rag corpus."""
     if vector_db is None:
-        rag_corpus.rag_vector_db_config = GapicRagVectorDbConfig(
-            rag_managed_db=GapicRagVectorDbConfig.RagManagedDb(),
-        )
+        rag_corpus.rag_vector_db_config = GapicRagVectorDbConfig()
     elif isinstance(vector_db, RagManagedDb):
         rag_corpus.rag_vector_db_config = GapicRagVectorDbConfig(
             rag_managed_db=_convert_rag_managed_db_to_gapic(vector_db)
diff --git a/vertexai/rag/utils/_gapic_utils.py b/vertexai/rag/utils/_gapic_utils.py
index 3ee39a7a0f..dfcd46989c 100644
--- a/vertexai/rag/utils/_gapic_utils.py
+++ b/vertexai/rag/utils/_gapic_utils.py
@@ -648,7 +648,7 @@ def set_backend_config(
 
     if backend_config.vector_db is not None:
         vector_config = backend_config.vector_db
-        if vector_config is None or isinstance(vector_config, RagManagedDb):
+        if isinstance(vector_config, RagManagedDb):
             rag_corpus.vector_db_config.rag_managed_db.CopyFrom(
                 GapicRagVectorDbConfig.RagManagedDb()
             )
@@ -668,7 +668,7 @@ def set_backend_config(
             rag_corpus.vector_db_config.api_auth.api_key_config.api_key_secret_version = (
                 api_key
             )
-        else:
+        elif vector_config is not None:
             raise TypeError(
                 "backend_config must be a VertexFeatureStore,"
                 "RagManagedDb, or Pinecone."