If someone can help with a XenForo extension for envolve, it's really been very good for us on vB. It's a little pricey, compared to arrowchat/cometchat, but the seamless offloading has been outstanding.
And to contradict the poster above, we've had *increased* forum participation since making an 'easy' chat available...And our users stick around a *lot* longer. We just about had a riot when we had to temporarily dump envolve (for CometChat, Arrowchat didn't have a hosted service)...
At the end of the day, a 'footer bar' chat is now a requirement for our community. It really is one of those things that seems to separate forums from communities.