There is a risk, but I know several people who had moderate to sever GERD and still decided to get VSG. I think it depends on your comfort with risk and that you may have to continue the same medication or perhaps more treatments for GERD in the future. A close friend of mine had extreme GERD, who is a nurse, still went with VSG. Her symptoms improved slightly, but she is still dealing with it like she did pre-surgery.
You've got to set your expectations realistically if you go VSG and have a history of heartburn/GERD. There is a good chance nothing will change or it could get a bit worse. From what I've read, about 25% of patients who get VSG who had mild to severe GERD got a bit worse, and 75% remained the same or got better. As far as risk goes, 25% is pretty high, but again, it all depends on your level of risk and knowing yourself that if you move forward and it gets worse, you knew it could happen and you might need a revision. That being said, there is still a greater chance that things will remain the same or possibly improve. I personally don't know that I'd take the risk, but I can't say for sure since I wasn't ever in that position to make the decision.
You might seek out a second or third opinion from other surgeons.