There are a few ways to reduce the high load problem while using a vbulletin forum. Install the following cacher plugins which will reduce the load dramatically a lot for your forum:
http://www.vbulletin.org/forum/showthread.php?t=107315
http://www.vbulletin.org/forum/showthread.php?t=107615
http://www.vbulletin.org/forum/showthread.php?t=110144
http://www.vbulletin.org/forum/showthread.php?t=111560
http://www.vbulletin.org/forum/showthread.php?t=82628
