I know it's too late to help the original poster, but for anyone else that has this problem and stumbles across this...
If you think about the flash, the lens, and the camera as having a relationship, it becomes an easy problem to solve. The glasses are a mirror. If the flash hits the glasses in a way that bounce into the camera, you'll get a bad reflection. If not, you're fine.
So, straight on? Bad reflection. Flash high, camera low? Bad reflection. Flash right, camera left? Bad reflection, etc. Think of the light from your flash as a ball on a pool table, shot from the direction of the flash. The glasses are the wall of the table, and as long as your camera isn't in a place that the "ball" (light) hits, you're fine.