I've never known complications from surgery to be genetic. Diseases that may result in needing surgery being genetic, yes, but not complications. It sounds like they are concerned. My dad went through the same thing. Tried to talk me out of it, but months before asked my mom if she thought I could benefit from it. Now he's giving me advice on how to do well. (he's never had any kind of surgery in his entire 56 years!)
When I made the decision to do this... and stuck to that decision, there was nothing anyone could say or do that would change my mind. We don't (or at least we shouldn't) be doing this to please anyone but us.
You may want to try to sit your parents down and educate them on VSG...or take them to a support group meeting (idk if yours allows family members, the ones at my hospital do). Print out some articles and let them read up on it.
God bless you on your journey! Your parents will come around!
Lauren