Does Vanquish treatment causes skin to loosen? I am on my 3rd treatment and my buttocks are sagging.

Vanquish ME (the newest Vanquish treatment) kills fat cells in a non surgical fashion. As the fat goes away the skin generally retracts with it. Most people over time also get a small amount of skin tightening from the heat on the skin. For more skin tightening we often combine Vanquish ME treatments with Exilis Ultra 360 treatments to both reduce fat and tighten skin. The Exilis treatments can either be alternated with the Vanquish or done after the Vanquish treatment is completed. Remember that the skin tightening takes longer to see the final results than the fat reduction.

One of the best features of Vanquish ME is that it also gives moderate skin tightening along with fat reduction when compared to some other technologies.  Sounds like the fat loss has preceded the skin tightening.  If it doesn't improve, you may consider a procedure such as Exilis Ultra, Skintyte or even Ultherapy, Thermage or ThermiRF to tighten the skin up and give more contouring.  Skin tightening can take up to 3 months or longer sometimes.