diff --git a/litellm/llms/vertex_ai/common_utils.py b/litellm/llms/vertex_ai/common_utils.py
index 6d050d5a856..63015de855d 100644
--- a/litellm/llms/vertex_ai/common_utils.py
+++ b/litellm/llms/vertex_ai/common_utils.py
@@ -620,6 +620,9 @@ def _fix_enum_empty_strings(schema, depth=0):
     if depth > DEFAULT_MAX_RECURSE_DEPTH:
         raise ValueError(f"Max depth of {DEFAULT_MAX_RECURSE_DEPTH} exceeded while processing schema.")
 
+    if not isinstance(schema, dict):
+        return
+
     if "enum" in schema and isinstance(schema["enum"], list):
         schema["enum"] = [None if value == "" else value for value in schema["enum"]]
 
@@ -633,6 +636,14 @@ def _fix_enum_empty_strings(schema, depth=0):
     if items is not None:
         _fix_enum_empty_strings(items, depth=depth + 1)
 
+    # An Optional[Literal[...]] arrives here as an `anyOf` branch, because
+    # convert_anyof_null_to_nullable runs first. Without this arm the empty string
+    # survives on optional fields while the same enum is fixed on required ones.
+    anyof: Final = schema.get("anyOf", None)
+    if anyof is not None and isinstance(anyof, list):
+        for item in anyof:
+            _fix_enum_empty_strings(item, depth=depth + 1)
+
 
 def _fix_enum_types(schema, depth=0):
     """Remove `enum` fields when the schema type is not string.
diff --git a/tests/unit/llms/vertex_ai/test_vertex_ai_common_utils.py b/tests/unit/llms/vertex_ai/test_vertex_ai_common_utils.py
index 04a7ee451c4..ba5a5a23cc6 100644
--- a/tests/unit/llms/vertex_ai/test_vertex_ai_common_utils.py
+++ b/tests/unit/llms/vertex_ai/test_vertex_ai_common_utils.py
@@ -1755,3 +1755,67 @@ def test_get_vertex_ai_lyria_model_info_is_none_for_non_lyria_speech_models(mode
     assert get_vertex_ai_lyria_model_info(model=model) is None
 
 
+def test_fix_enum_empty_strings_nullable_enum():
+    """An Optional[Literal["", ...]] field must be sanitised like a required one.
+
+    convert_anyof_null_to_nullable runs before _fix_enum_empty_strings, so a nullable
+    enum has already become an `anyOf` branch by the time the sanitiser sees it. Without
+    recursion into `anyOf` the empty string survives on the optional field only.
+    """
+    from litellm.llms.vertex_ai.common_utils import _fix_enum_empty_strings
+
+    schema = {
+        "type": "object",
+        "properties": {
+            "mode": {"anyOf": [{"type": "string", "enum": ["", "fast", "slow"]}, {"type": "null"}]},
+            "plain_mode": {"type": "string", "enum": ["", "fast", "slow"]},
+        },
+    }
+
+    _fix_enum_empty_strings(schema)
+
+    assert schema["properties"]["mode"]["anyOf"][0]["enum"] == [None, "fast", "slow"]
+    assert schema["properties"]["plain_mode"]["enum"] == [None, "fast", "slow"]
+
+
+def test_fix_enum_empty_strings_anyof_without_a_null_branch():
+    """The gap is not limited to nullable fields - any enum under `anyOf` was missed."""
+    from litellm.llms.vertex_ai.common_utils import _fix_enum_empty_strings
+
+    schema = {
+        "type": "object",
+        "properties": {"m": {"anyOf": [{"type": "string", "enum": ["", "a"]}, {"type": "integer"}]}},
+    }
+
+    _fix_enum_empty_strings(schema)
+
+    assert schema["properties"]["m"]["anyOf"][0]["enum"] == [None, "a"]
+
+
+def test_fix_enum_empty_strings_tolerates_a_non_dict_anyof_branch():
+    """A malformed branch must not raise, matching _fix_enum_types' isinstance guard."""
+    from litellm.llms.vertex_ai.common_utils import _fix_enum_empty_strings
+
+    schema = {"anyOf": ["string", {"type": "string", "enum": ["", "a"]}]}
+
+    _fix_enum_empty_strings(schema)
+
+    assert schema["anyOf"][1]["enum"] == [None, "a"]
+
+
+def test_build_vertex_schema_sanitises_a_nullable_enum_end_to_end():
+    """The whole pipeline, as a tool schema reaches it from pydantic."""
+    from litellm.llms.vertex_ai.common_utils import _build_vertex_schema
+
+    parameters = {
+        "type": "object",
+        "properties": {
+            "mode": {"anyOf": [{"type": "string", "enum": ["", "fast", "slow"]}, {"type": "null"}]},
+            "plain_mode": {"type": "string", "enum": ["", "fast", "slow"]},
+        },
+    }
+
+    built = _build_vertex_schema(parameters)
+
+    assert "" not in built["properties"]["mode"]["anyOf"][0]["enum"]
+    assert "" not in built["properties"]["plain_mode"]["enum"]