My informant is the daughter of Vietnamese immigrants. She frequently visited Vietnam during her childhood, and told me about this superstition that her mother warned her about.
One time as a child, my informant was sleeping in the same bed as her parents and brother, and decided to sleep in the reverse position on the bed; with her feet near the pillow and her head near the foot of the bed. Her mother refused to let her sleep this way, and told her that if she slept in that position, that demons would grab her and take her away in her sleep.
I have noticed that this is a belief in other Asian countries as well, and I find it interesting that there is a connection between sleeping in an unusual way and supernatural vulnerability.