Would it be bad to have a BBL 5-6 months after having VSG done?

You should probably obtain a stable weight for at least 6 months following weight loss surgery.Kenneth Hughes, MD

Dear finelikewine,the optimal weight to have BBL is a very common question and for good reason. Having surgery before you reach your target weight can lead to suboptimal results if you were to lose more weight after surgery.
Good first questions to ask yourself are:
1. Am I at my goal weight?
2. Is my weight stable? have you lost or gained more than 10lbs in the past month? What was your weight last year?
3.  Are you eating a healthy diet and exercising?
If you are not at your goal weight or your weight is not stable, you should hold off on surgery.
If not, great! I would suggest you to consult a board-certified plastic surgeon. You will get the best recommendations after a thorough examination.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

Ideal candidates are at a stable weight,  in good health , and have excess fat in some parts of the body and too little in other regions.  Gaining weight before the procedure is to no advantage and when you lose weight again the fat cells will shrink just like the donor site fat cells.  Typically, the transferred fat results in a significant increase in volume of the body site being treated.