What are the risks of vacuum therapy - non-surgical buttocks enhancement?

It is very difficult to determine the exact size and shape implant you will require to best match your ideal butt image without an examination by a board certified plastic surgeon. Not just any board certified plastic surgeon, but one with many years of frequently performing breast augmentation surgery including different approaches, techniques and implant choices. This is because several measurements not to mention your breast characteristics are needed to determine the optimal implant size to obtain your goals. Without knowing these dimensions it would be difficult to make this determination. For example, the existing base width of your buttock will determine, in many cases, the maximal volume per implant profile that you can accommodate. Liposuction of your lower back and fat transfer is also helpful as an adjunct to implants. Using “want to be” photos however are useful if simply provided to the surgeon.

 Sounds like a gimmick to me and indeed the reason you are unable to find any "scholarly research online".  I can gaurentee it will produce minimal results and very short lasting results, definitely not permanent. Also beware of other "non-surgical buttock enhancement" methods (see below). 
 Like any booming fad or fashion, there are always scammers looking to get a peice of the action and make quick/easy profits.  Well as the saying goes, "anything too good to be true, usually is (not true)". Unfortunately the ripe new world of buttock augmentaion has become the next victim with a bunch of nonsense magical creams, potions, lotions and pills...NONE OF WHICH WORK AND ARE NOT FRAUGHT WITH SERIOUS SIDE-EFFECTS/COMPLICATIONS. It is not yet possible to take a substance that will only and exclusively makes your derrière grow. Other than being genetically blessed in the buttock and/or performing consistent rigorous exercise, there is no "natural" way to grow the buttock other than surgery.