tleigh and Geneticdrift, do a search and you'll find much discussion on the subject of running single/multiple bookshelves as centres and I think those that have tried have been happy.
I am currently running a single M22, vertically, as a centre and I'm extremely happy with it. No matter what ones position is in the room the centre channel is perfectly anchored to the centre. The blend from L C R is flawless and better than any system with a designated centre that I have heard. I have not had an opportunity to hear the VP100 or VP150 so can't comment per a comparision but there are many happy VP owners out there and a number of +ve reviews on them particularily making the point of how well they blend in a sytem compared to other centres.
Aesthetics is a major problem with this type of centre for many and a valid enough reason not to do it. But if looks aren't a problem then for sonics I feel that a single M2 or M22 is the ultimate and very much worth considering; particularily, if the traditional approach isn't giving the expected results.
Good Luck
EDIT:
Here's
one of many good threads on the subject
