There is a difference between a shapely bottom and a big bum. Back in the day, women used to ask "Does my bum look big in this?" If it did, they would scream and change outfit. Fast forward years later, when the same question is asked, we want the person being asked to respond, "Yes it does, and it looks fabulous darling."
The risks associated with ordinary liposuction all apply too, plus there's an added risk with Vaser Lipo of second- or third-degree burns, thanks to the use of ultrasound waves to emulsify fat in the deep and superficial layers of the skin (standard liposuction uses other methods of fat cells break-down).