Yes, this has also happened to me before. There may be some sort of inactivity timeout (possibly a default setting). If I have a long post, I usually type it in a word processor (like MS Word) first so I don’t lose anything (I’m assuming you have a word processor) and then paste it into the vBulletin editor. This is a little more trouble, but it saves losing everything you typed, especially if it is a long post! I have also had another issue with the vBulletin editor (chops off the last part of the post). I’m working with the admins on this now.