I'm not queit sure what you're trying to do, maybe give us a few more details. But simply making hostname aliases for your domain in Apache's conf isn't going to make that hostname resolve. What I mean by that is that if your registered your domain as mysite.com, and you want forums.mysite.com to resolve to you, then Apache's conf isn't going to make that happen. You'll need to adjust your DNS server's configuration to include any all hostnames you want to resolve for the domain. All resolve requests on your domain are/should be answered by one of your DNS nameservers, if those nameservers don't know where forums.mysite.com is, then they'll simply answer that they don't know. If you have control over the nameserver, this is easy to fix by just adding a CNAME or A record to the domain's configuration file.
Anyway, the ServerAlias directive in Apache is for making multiple hosnames point to the same website, which seems to be what you want, but keep in mind that it must first get to Apache before Apache can do that, as explained above. More info on that directive can be found on Apache's website.