As someone who has experience with web design, the fact that you have a vBulletin specific chatbox is inconvenient. It only displays on one page, is clunky, and takes up loads of room. Naturally my first option in most cases is IRC since it's a dedicated chat system that works outside of my web browser, but as for on-site chatboxes you may want something minimal that can actually display on every page. From what I know, Chatango has chatboxes that you can have on your page as a tab and open them to expand, and those seem to work really well for designs that require as much room as possible in order to look correct.