There are after market add-ons, none directly from vbulletin that I know of.
It's not the deals, it's just the abandonment of the forum discussions that I worry about.
Steve, can you elaborate? I'm not following why you're concerned. The chat would only be for doing deals. For example, I have a gun for sale now, and I've received private messages. When I hover over the member's name, it tells me they're online. It would be nice to "Chat with buyer/seller" to make things go faster/smoother. I wasn't recommending an general chat feature, just for active classifieds. Might be more involved than I'm thinking though.