Vietnamese Buddhism has demons in it, inherited from local beliefs and from Chinese Taoism.  I think though the translation of "demon," like the translation of a lot of this sort of word, misleads.

Taoist demons, because they are so frightening in appearance, are often considered helpful in scaring away other demons and the like.  They are also generally believed to be the spirits (ghosts) of dead people who for some reason (mainly bad karma) reincarnate as demons.  There are a gazillion types you can come back as, depending on what you did wrong during your life.

Now there is an example of a translation that misleads -- dragon -- not at all the beastie of the West.

The best way to protect yourself from demons is with loud noises, such as drums and fireworks.  Statues of certain Chinese worthies and of course lion or dragon demons (especially) in your home are also protective, as long as you do the proper rituals.

The idea of demon possession seems to be a Western idea -- insane people are considered to have a medical problem.

Much of this, of course, is seen as superstition by most of us in Vietnam -- but we do like the Taoist rituals with the dragons and fireworks.