This is a problem with the way the vBulletin forum software works. I've encountered it too. So far, the authors of vBulletin have been able to lessen this problem (it used to be worse) but not eliminate it.This is an on going issue but when I type a post then edit, then try to do another edit the first edit disappears unless I reload. It is a pain to make 2 edits without having to reload the page. Can this be fixed?
I've never encountered the problem though I've done the steps using Firefox.
Are you using Safari?