You're not going to get a definitive answer here because manufacturers take different approaches to attaching the Gore-Tex liner, and that might affect the result you'll get. Your best option would be to ask Vasque directly.
Though it's probably safe to say that you'll end up with a non-optimal result. Even if the membrane isn't torn by the stretching, the micro-pores would presumably end up larger than they should be.
You should be aware that for the great majority of users, Gore-Tex boot liners fail anyway, and sooner rather than later if any real mileage is being covered. In fact, most experienced users tend to avoid Gore-Tex boots.
If the boots aren't comfortable they're not much use to you. So if you can't return them, I can't see that you've anything to lose by trying to stretch them. It may simply mean that the membrane will fail a little earlier than otherwise.