Why is it so cheap?
I do not own at Mountain Vista. But, I stayed at Mountain Vista about 2 months ago. Also, I do not ski so I am not an expert on ski resorts. (Since you are familiar with the Avon area, most of this post could be stuff you already know.)
First, Mountain Vista has no air-conditioning. This limits the desirability of the resort in the summer. Fans in the rooms worked OK when I was there in mid-September. In July or August, the rooms might not be comfortable.
Mountain Vista is furnished at the Sheraton, not Westin, level. The decorating is a little tired and the rooms are pretty gloomy. I expect that the level of the decorating will be taken care of by a special assessment. (BTW, I see no advantage in putting a whirlpool bathtub in the bedroom, rather than the bathroom. But, that's a personal preference.)
Compared to other Westin time share locations, my wife and I felt a little cramped. Storage (closet and drawer space) in the room was quite limited.
The new Westin resort and timeshares at Riverwalk in Avon is opening soon. It will have a gondola lift at it's door up the mountain directly to Batchelor's Gulch (not Beaver Creek). Mountain Vista is about 2-3 blocks away from the new lift. I have no idea if using that lift will be for free or for pay for Mountain Vista owners. Riverwalk is beautiful and posh. Mountain Vista is utilitarian, by comparison.
Staying in Avon is not as convenient as staying at Beaver Creek or Batchelor's Gulch. Avon is on the "flats" along the Interstate. Beaver Creek and Batchelor's Gulch are way up the mountain where the slopes are. Avon is a sleepy little town without much charm, IMO. Avon is definitely not Vail, but it's obviously trying to become Vail-like. Maybe it will succeed; maybe not. (BTW, Avon is financing the push to become another Vail partially through a real estate transfer tax. Since I am not a skier and not interested in buying in Colorado, I did not inquire about how that tax works.)
In the end, I must say the staff at Mountain Vista was extremely friendly, well-informed and helpful. My wife and I found the concierge to be helpful and had a good time there.
I don't know if the disadvantages of staying on the "flats' of Avon outweigh the advantages of staying further up the mountain at the two resorts or whether the new Westin is depressing prices at Mountain Vista as people who own at mountain Vista make an "upgrade" to Riverwalk. But if you want a "no frills" ski experience in the middle of a charmless town populated by banks, strip-like malls and adequate restaurants and want to buy at an older, "voluntary" Starwood resort because you will visit every year, Mountain Vista may be for you. For skiers, the great ski slopes that are nearby may trump everything else.