can you take a pic of yours? does the tention pulley lign up with the other pulleys and have you checked the belt for rips or splits?
The most common problem with the tensioner pulley is that the bolt that holds them loosens up and they can become sloppy. Or they have worn the bolt hole out, the end result is the tensioner pulley can move or become a little sideways which will allow the belt to
walk off the pulley when the engine is turning.
Check your pulley compared to the other pulleys and make sure it is aligned strait with the others. Also check to make sure ( I believe it should be a spring device on VW to keep the tensioner pulley tight against the belt ) the tensioner pulley still can put enough pressure against the belt to keep it in place.