Is it permissible to offer the tahajjud prayer after the witr prayer?

It is permissible to offer a voluntary prayer after the witr [the final prayer at night] prayer. However, it is desirable that the night prayer be ended with witr. Therefore, it is desirable for a person who thinks that he will most probably be awake at night to offer tahajjud, to offer witr after it. But a person who fears that he might not wake up, should offer the witr prayer before going to bed.
And God the Almighty knows best.