reduce the number of queries by removing any unnecessary hacks.
vbPortals is known for adding a huge amount of queries to your main page. the optimal no. of queries for a board is max. 40. after that you start to get your server on the red side. immagine that you have 1000 users and they proccess 60queries at the same time. that's 60,000 queries in the same fraction of time. that can double easy if they do a mix between the frontpage (average 90 queries for vbPortal) and vBulletin.
wajones did an excellent job with the portal, but in my personal oppinion i think it puts alot of stress on the server.
try this, place {getmicrostats} in your homepage. let me know the results.
also keep in mind that an unhacked board have only 17queries.