Why is Vaser liposuction risky for removing some types of belly fat?

Liposuction of any type (Vaser, SmartLipo, PAL, or "regular" liposuction, liposculpture, etc.) in the belly area treats fat beneath the skin and above the abdominal wall (muscle and fascia). Not every smooth-skin protruding abdomen is appropriate for liposuction, since in SOME patients the fatty bulge is due to intra-abdominal mesenteric and omental fat, not a thick subcutaneous layer.

The subcutaneous fat layer appropriate for liposuction is only one type of fat, and Vaser is not only NOT risky, but IMHO one of the best modalities for liposuction of the belly area. (Vaser ultrasonic liposuction and "liposculpture" is what my partner and I perform routinely for the belly area, as we feel this offers the safest, most thorough, and smoothest fat removal of all of the available options out there). Many other options exist and can work well also, so if you are getting (wrong) advice that Vaser is "risky" then you should read between the lines.

This "surgeon" may not be an ABPS-certified plastic surgeon, but a doctor of other specialty training that may have little or nothing at all to do with plastic surgery, or this doctor may not own a Vaser and therefore "talks it down," or may just be uncomfortable performing this operation on you (but is somehow unable to state that directly).

Simply put, this is an incorrect assertion, so I would question why you have been told Vaser is "risky" for "certain types" of belly fat.

Vaser liposuction, also known as Vaser #lipo, LipoSelection or Liposuction, is an advanced body #contouring procedure that selectively removes unwanted body fat. An alternative to the harsh techniques of traditional liposuction, #Vaser lipo uses state-of-the-art ultrasound technology designed to gently #reshape your body. There are three types of #Vaser Lipo. Each benefit patients differently depending on their needs.What distinguishes the Vaser lipo procedure is its ability to differentiate targeted #fat from other important tissues; such as nerves, blood vessels and connective tissue. Innovative VASER #technology breaks up fat while preserving these other important tissues to promote smooth #results and rapid healing.All surgical procedures carry some degree of #risk and the need for revision surgery is always a possibility. Some side effects and complications associated with #liposuction include: Hematoma (#bleeding), blood clots, fat clots, infection,  deep vein #thrombosis, cardiac and pulmonary complications, excessive #fluid loss, fluid accumulation (#edema), leg #swelling, sensitivity to #sensation, #pain, severe #scarring and poor incision healing damage to nerves, blood vessels, muscles, lungs, and abdominal organs is possible. 
Other side effects which may be experienced while healing which can cause concern are #swelling, #pain, #asymmetry #uneven skin pigmentation, discoloration of the #skin, uneven contours and rippling of the skin in the area(s) treated.
Hence, it is imperative you select a plastic surgeon who is #board-certified and has a great deal of experience with #vaser #liposuction. Aside from checking board-certification, it is suggested that you look at before and after photos of the surgeons actual patients, and read patient reviews. Gathering all of this information will help you make a well-informed decision. Always discuss your issues and concerns with the Plastic Surgeon during and following your consultation. Information that you read online may not apply to your personal situation.

You would have to be more specific.  We remove a great diversity of fat and the only unsafe fat would be fat in a hernia.  That fat we would like to leave where it is.