take your time and be careful, make sure channel of the glass frame is cleaned of all old adhesive first and use silicone like previously said, silicone will allow slip and will seal when cured, i think at the factory they used the black urethane sealer for this, that way it would also match the black color of the frames,
its 5 bucks a normal size caulking type tube for the bvlack urethane at any glass shop, use it for windshields and such also and to hold back all weather stripping for door glass runs and so forth
i cant believe youre breaking a vent glass trying to install it, thats crazy they are hard to break,
take your time and clean everything and make sure lips of the frame are not pinched or anything anywhere, inspect closely, fine detail is everything and will prvent disasters when doing anything ever,.
good luck