I haven't been in a couple of years now, but Traffic in LA used to carry a fair bit of Vivienne Westwood for men. It was always very interesting to me, but not being rich, I always tended to use my designer alloted money to Costume National, Paul Smith, and Helmut Lang coats and jackets that were a lot more versatile. I can think of a few obvious reasons that Vivienne Westwood does not go over well with American audiences:
1) Americans, including fashion mavens, like their cuts very clean and flattering. So brands like Paul Smith do a lot better than Yamamoto and Westwood, both of whom favor texture, pattern, and drape over structured silhouettes and simple patterns.
2) In the same vein, the whimsical, exaggerated details of VW clothes (contrasting buttons, exaggerated collars) do not have a receptive an audience
3) There is a market for more whimsical clothing, but VW's prices are too high for that customer, who buys instead from much more accessible brands like United Bamboo and Trovata (a lot of their stuff looks like a poor Man's V.)
Anyway, I could be wrong on one or more counts, but those are, for me, the obvious reasons the label does not do well in the United States. I wager that it does poorly in Northern Europe and Canada as well, where extremely austere styles are favored.