Using the VASER liposuction, you don’t lose any significant amount of fat transferred to the buttocks. As long as you use VASER fat harvesting, you should be fine to sleep and sit after the procedure. VASER has a 95% viability of the fat cells and retention of the volume. Thus, you do not have to worry about having a tummy tuck and sitting for extended periods. 

In summary, you can perform your BBL and tummy tuck at the same time.