The big name search engine bots tend to be very respectiful and rarely ever cause problems...
It's people using automated programs to download much of a website so they can then read it off-line, search through, archive, etc... most of these are not website friendly and will attempt to request numerous pages all at once, which will totally swamp most vBulletin forums...
Throttling such excess requests is quite challenging for small forums that don't have much hardware behind them - one quick way is to use IPTables and temporarily drop all incoming packets from the abusive IP(s).
Ron