alex, based on your last php.ini configuration, you should think about using op-code caching like mistwang suggested. For php 4 code, I would highly recommend eacclerator.co.uk. Note when installing, make sure you disable "code optimization", only enable the caching part of eAcc.
Opcode caching can drastically improve php's cpu usage.
Another sure thing is also to upgrade to the latest vBulletin build.