1. I have never seen this before with vBulletin so I can't comment.
2. vB allows you to prune, delete, manage everything that's enterd into your database. See what you can do at
http://www.vbulletin.com/admindemo.php
3. vB3, by default, is very optimized to be very fast, it takes a load of the server from vB2, even on very large boards.
http://www.vbulletin.com/forum/showthread.php?t=47430 gives you a list of the biggest vB boards and you can see how such a big forum is fast