FWIW, any 12" toilet will fit on a 14" RI, but, you'll have an extra 2" behind the toilet than originally designed...functionally, it will work just fine, though.
As said, the Vespin (and any of the Toto toilets that use the Unifit adapter) will work, and will fit better if you replace the 12" Unifit adapter that comes with it for the optional 14" one.
Most 14" toilets just use a deeper tank and are not purpose built for a 14" RI, so they stick into the room that extra 2"...the Totos that use the Unifit will sit the same distance from the wall and are actually exactly the same toilet for the 10, 12 or 14" RI...just the UniFit adapter changes.