If you do an ILS - then you should be in the middle of the VASI / PAPI.
Not necessarily so. The siting of the ILS glide path transmitter may not coincide with that of the VASIS/PAPI installation. In addition, some PAPI/VASIS light signals are set for long body types with typically a 71 ft TCH, whereas normal ILS TCH is 51 feet.