It just so happens I help run one of the largest vB4 sites (14Mish posts)... you might find this useful:
http://articles.digitalpoint.com/con...ze-vBulletin-4
Took our vB4 forum servers from being strained to almost running idle with the same traffic.
Also, in case you have not found out yet... search in vB4 is almost unusable for a large forum (even after you finish the indexing process). We built a Sphinx-based search system that is fast as hell (search for "paypal" in our forum as an example... which exists in well over 1M posts).