I think I used to say vee-stah, but that's because the word looked like a Latin-based word to me...I think being around Illuminati, I've unconciously switched to saying Vis-tah.
I actually have a hard time saying Vis-tah.... but that's what you hear on commercials...
I personally relate it to the spanish word vista (view).... and that is pronounced vee-stah. So that is why so many are pronouncing it that way... So, in effect, I think M$ doesn't know how to pronounce their own product!
I believe it's supposed to be pronounced vis-tah but I pronounce it vee-stah because I have a slight speech impediment (I stutter) and, as Illuminati pointed out, vee-stah just rolls off the tongue easier...
I suppose you can safely call it vis-tah, Dan, judging by the votes, and Bill Gates. If you look at some interviews with Gates, he pronounces vista vis-tah.