The vex, as we all know, teleport and travel through time. It is reasonable to assume that they can travel to any destination they were at int he past, or will be in in the future. therefor, i beleive that vexdont teleport forwards or backwards or anything, they are just travelling into the future, giving us the impression that they are teleporting. they could travel to where they will be, so instead of walking to where theyll be they just go into the future to save time.