This just happened to me! 1.7.1.5 as well. I was climbing a ladder to a hospital, got to the top of the ladder, couldn't get on the roof. I tried pivoting myself to get loose and fell to my death. I respawned prone under some building and can't get out. I tried rolling/crouching/standing still stuck prone. I can't vault out like you apparently. HELP!! Edit: I have no idea where I was when this happened or where I am now after respawning :(