Though I am not familiar with the backend of vB forum software (which is odd because my job is as a Web Communities Administrator), I'm going to guess that using a vB integrated chat plugin would be the best way to keep out the jerks. Very few idiots are going to go through the trouble of registering with the board just to hop onto a chat and be annoying. And even if they did, any admin or moderator who was also in the chat could easily kick/ban them from the chat and/or site.
IRC is a pain for a lot of people who are too used to the ease of java chat type systems. Plus, I have experienced more troll problems in IRC chat rooms than I have ever seen with forum integrated clients.