The adapter plates for the rear speakers in my old 2600 VDP were fixed in with screws, not looked at the Vitesse yet. The speaker sizes are approx 4" or 10cm. Have got some 10cm Sony speakers in the front(which were a straight swap for the sh*te old speakers, but not bothered with the rears as I've got a pair of 6" x 9"s in the rear parcel shelf.
Also, I used new cables for all of the speakers, routed the same way as the main loom, as the original wiring goes through that crappy fader on the centre console, plus the sound quality is awful through the original cables......