diff --git a/litellm/llms/vertex_ai/common_utils.py b/litellm/llms/vertex_ai/common_utils.py
index 52345e1ff40..7ff61254298 100644
--- a/litellm/llms/vertex_ai/common_utils.py
+++ b/litellm/llms/vertex_ai/common_utils.py
@@ -684,6 +684,9 @@ 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"]]
 
@@ -697,6 +700,13 @@ 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.
+    if "anyOf" in schema and isinstance(schema["anyOf"], list):
+        for item in cast(list[object], schema["anyOf"]):  # cast-ok: an untyped JSON list
+            _fix_enum_empty_strings(item, depth=depth + 1)
+
 
 GEMINI_MAX_ENUM_CHARACTERS: Final = 2000
 _GEMINI_EXACT_INT_MAX: Final = 2**53 - 1
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 32d68249858..a7ac0cd76a1 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
@@ -2728,3 +2728,69 @@ async def test_vertex_ai_token_counter_routes_like_generate_content(
         )
 
     assert (None if result is None else result.total_tokens) == vertex_counted_tokens
+
+
+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"]