For vBulletin there is an addon, that allows you to mention someone the Twitter way (like @username). That triggers a notification in the notifications box (at least that's what I heard). But that's not ideal. Because that will make people just use that (@username) instead of proper quotes, making it harder for others to follow the conversation.
I know this is kind of a blurry request, but I thought I'd throw it out there. If anyone can make something sensible out of this xF's devs can!