The VR is engaged immediately, same as with the default shutter button half press. It will stay engaged while the AE-L/AF-L is pressed. If the back button is released, the VR will disengage within a second. It still re-engages normally with a shutter half press.
Best of both worlds, seems to me. If you are tracking a subject the back button is likely to be pressed all the time, in which case the VR is engaged all the time and that small fraction of brain monitoring motor control of the shutter finger for a half press can be reassigned to the task of tracking and choosing the moment for a shutter press. Otherwise, if you are simply picking up focus for a landscape shot and releasing the button, the system works just like any other VR-aware body.