5503 is not enough, 5506 is the commit solving this problem.
No need to bury into this deeper, the reason why it was not working with WithDeepCopy is apparent (WithDeepCopy<Vector<T>> is not the same type as Vector<T> and template specialization for Xmlize was defined for Vector<T> only).
It is now fixed by moving Xmlize as Vector method, thus completely avoiding template specializations.