I got 2 vac bags in this years Mistic Gear Box DLX 170 cm, with a small pump included.
I never used them, because I didn't have any space issues with this bag
Ussualy I have problems with the weight. A couple of 100 g here, a couple there... I always have to live somethin behind and don't even consider using vac bags for this reason, that goes for kite backpacks to.
Shouldn't make any damage to my foils
p.s.: they might come in handy, if one day I decide to make a foilboard. It's a great size to vacuum a foil