www.subdomain.domain.com is basically interpreted as a separate subdomain from subdomain.domain.com. If you want both to work I think what you need to do is basically duplicate your "forums" subdomain and make a "www.forums" subdomain with the same settings as the former. You should be redirecting one to the other though to avoid duplicate URLs. Allowing www at all for your subdomains might be considered bad practice, however - I'm not sure.

As for your main problem, just to be clear, we are talking about the quick style chooser in the footer, right? That script isn't supposed to do anything to your url apart from setting the styleid variable in it. What version of vBulletin are you using? Can you go into the footer template and copy the style chooser part and paste it here so I can see?