The difference is the sound and most important the feel of the keys.
If you play the VK8 you get a WOW feeling, you can't stop playing real organ stuff à la Jimmy Smith etc. The VK7 is nice but that's it.
My opinion is only based at playing the VK7 at the Frankfurter Messe 2001 and the VK8 recently.
I hope I made myself clear
Rene