Do the vietnamese believe in ghosts?

Answer 1

No, belief in western type ghosts is not part of Buddhism. What some people, who are incidentally Buddhists, believe is of course up to them.
Yes, many Buddhists would accept the idea of formless beings like ghosts.

Answer 2

Buddhism holds the belief of reincarnation and hence most of the times the dead get reincarnated and are re-born which does not allow anyone to speak to the dead. However, there are cases where when the dead are reincarnated as ghosts they do tend to talk to the dead and also in cases when a person has died recently they speak to their family members once before leaving.

Answer 3

They believe in genies.

Their folk religions is called 'Geniism.'