[shiva]

San-Deep: That's a hack I believe, I use the same thing, and it's available at the VBaC homepage as well.
Ghost, can't help you, as I don't know what you've put on your site, but the first thing you should look at are the modules. Some take up a lot of server resources, things like shoutboxes especially, and you may have over did it a bit. Deactive some modules and experiment a bit to see what's the best combination of features you really need, and the server speed. Support at VBa is excelaant, they will answer any questions you have on that. It's generally overuse of modules in combinations that slows a site down.
Beyond that, it may be some settings in VB AdminCP that you may have to adjust, most of them are clearly marked in the descriptions when you set the vboptions. Pay close attention to them, when they say that something may impact server resources, they generally mean it.
VBaCPMS does add some overhead, but is quite good overall, better than say a average PHP-Nuke site, and far superior to other CMS type systems out there for VBulletin, IMHO.