What happens when you try "2dWireframe" or "Shaded" <<== note capitalisation.
also,
Have a look at
Autodesk.AutoCAD.Internal.Utils.
public static ObjectId visualStyleId(string visualStyleName);
and
public static void SetCurrentViewportVisualStyle(ObjectId visualStyleId);
public static ObjectId GetCurrentViewportVisualStyleId();
and
public static string visualStyleName(ObjectId id);