I know I am coming in on this a little later. However, I have some relevant information which involves session times and cookies. If you're using firefox, go to Tools > Options > Privacy and make sure your cookies are set to expire for "When they expire" and that you've enabled accepting cookies.

That aside, FA, I myself have noticed a session issue with the website. Now, I don't quite know if this is something you can change with vBulletin itself. Rather, you'd probably have to talk with your host or edit the session timeouts yourself. I usually resort to posting 1 or 2 videos at a time as the magnitude of selecting for example, 10 files and clicking Upload only to timeout before you can submit the thread, thus losing every attachment.

I guess one workaround is to not let users upload as many files as they want at once. Another could be retaining attachment data so that it's not lost between login/logout. Don't quite know if the last part is possible through any modifications to vBulletin, but it'd sure be nice as I've lost a lot of posts this way.