I've never used vaseline, but a teaspoon of vegetable oil. Lay the background flat, spread the oil all over. Carefully line up to the tank, and use a straight edge (credit card, hard plastic ruler, etc) and work all the bubbles to the edges. The oil holds the background in place, makes it appear much crisper, and you don't get the water drips. Works best using brand new, unstretched background--no weird bulges to deal with. It's worked for the SW tanks for about 2 years now.