The benefit of running memcache on the same machine as the Apache/MySQL used by vBulletin is debatable. Linux and MySQL already do a pretty good job of caching things; by using memcache you're just taking memory away from them.

Running memcache on another box (preferably one with a low latency to the web server) makes sense though.

On a side note, there's a useful vbulletin-specific caching system at  http://linuxbox.co.uk/vbulletin_caching_with_lbcache.php