Hollow Vibro Extension by Doc Johnson. This extension is something extra for the guys out there, the hollow vibro extension is a vac-u-lock compatible penis sleeve with vibrating action. Regardless of your erection, this sleeve will add your love tool extra stamina and hardness, while exciting you with its multi-functional vibrating feature. This dong can only be used with a vac-u-lock harness with snaps. More Penis Extensions for your add length!