Tribadism is not necessarily rubbing vaginas (which would be difficult anyway as they're internal, it's the vulva that's outside) - it's two women rubbing against each other. More often we do it thigh-to-vagina (sort of), which is probably where the idea for the thigh harness came from - it's a strap-on harness for dildos that is placed around the wearer's thigh. (And if you want to know what we do in bed, the answer is: "anything you can think of.")