I know some people subscribe to giving them something soft to land on. Personally' I think if your suction is adjusted properly, it's not necessary. Also, you have to be careful on what you use, there's the possibility the bees will get caught up in it.
Did you bring the Vac in the attic? If it was hot, that could be the problem. Did you keep the Vac running the entire time that the bees were in the enclosure?...You don't want to ever turn the vacuum off and leave the bees in the enclosure, they will over heat quickly.
Short of that's I would have to think your suction was too high.