Lawrence; i think the community users I've seen post so far are mature enough to stick to a comparison between products and xenforo. remember, they all run a site and xenforo is an option for them to consider.
And some negativity about vb is probably natural, as quite a few are displeased with it at the moment. But I think while some discussion about competitor products is healthy, it's not within the spirit of the community to start bashing them. Xenforo isn't such a place.