I'll throw my 2cents out there as I have done some testing with my M80s and an external amp with the 3808.
I could hear a difference when running the external amp and the 3808 vs. the 3808 alone. I was using Axiom's A1400 and when calibrated to the same SPL levels I could hear the details better, bass had more punch/fullness and the highs were more detailed when running with the A1400. Now my low volume was 65-70db any lower than that and the differences were MUCH harder to hear, if any at all, but as the volume increased the A1400 really shined, YMMV
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I would say run with the 3808 for awhile and see if you think the extra money for the exteranl amp(s) is worth spending money on. I know if I ever have enough spare change I am going to be buying an external amp some day but till that day comes I am not worrying about running with only my 3808, it still sounds ~95% as good IMO.
So far as the center channels go, running 3 speakers the same across the front is best, next would be some other vertical center that nearly matches the mains and lastly would be a horizontal design. I have done some rudimentary testing with my M80s, VP100, VP150, M22s and even a M60. I liked my M80s with the M60 the best, I never had an opportunity to run a third M80 which would, I suspect, be even better IMO
. I have run dual M22s as a center but situated to the sides of the screen and I wouldn't recommend this as the nearest speaker to the listener is where the center dialogue comes from and not from the center of the screen. Dual centers work when run above and below the center of the screen.
So far as the idea that the VP series can't keep up, well that is a matter of opinion and what you expect out of the center. The VP series works extrememly well for what they were designed for, HT use and mostly dialogue. For full range listening as in multichannel music, the VP series still works well but you can tell it isn't the same as the M80s but this is getting extremely nit-picky, I believe most people wouldn't notice the difference.
