A couple months I was trying this out for a client and I am not sure if this is the chat I am thinking about but I believe this does not integrate well with the Vbulletin user data. I believe people need to create a separate account when they want to chat.
You may want to double check this though.
I have 2 very large client sites using 123flashchat using their server and it has been ok though not perfect and support is non existent but that was the best option we could find for their purposes.
If you need something local you can get the
mngcchatbox pro free here. Nick released it free and you can set it up pretty good as a chat room instead of a chatbox. I am not so sure you can have a popup window though it could probably be coded without too much trouble. I am not sure on that though.