Really, there are only two ways to stop VGA:
1) start several new grading service to compete. From what I hear, VGA's profit margins are extremely low because they hand-make all the cases. If they had even a little bit of legitimate competition, I don't think they would be able to survive.
2) Prove that their service is a fraud. The best way to do this is to reseal a game (taking pictures of you doing it), sending it into VGA and successfully getting it graded, then post a video of you opening up the graded game. I don't think they would be able to survive that.