firstly Yes its possible using a redirect, but more importantly why cant you get the subdomain to work? What issues are you encountering?
A little more detail and I can help you with the best solution.
If you are getting "forbidden" you can rule out DNS issues. It would appear you have a configuration problem. most likely you haven't defined a default page and fancy indexing is disabled.
Try adding the folloing to httpd.conf
and make sure mod_autoindex is loaded
Take a look at your virtual server config for the subdomain. compare it to the one for your main domain.
Remember that virtual servers override the main configuration, In other words if something is defined in the main config part (anything outside ) it remains in force for your virtualhost. so either change it in the main section of httpd.conf or add it to your virtualhost.
Perhaps if you post it your config (just the relevant parts such as the virtual servers) we can suggest alterations
Your Virtualserver config looks fine.
Without seeing a lot more of your configuration I would hazard a guess to say the problem is with the forum software. if you can enter the forum via localhost/forum/index.php but not forum/index.php Then I would say that the forum software is configured to use an incorrect domain name.
Probably it was setup to use localhost rather than forum as the domain name. and probably it has also been configured to use the directory /forum/ rather than the root folder
This is just guesswork as its hard to determine the exact cause without a closer look.
If you create a file called /forum/test.html can you access it using http://forum/test.html ?
You may need to rename any .htaccess files to disabled.htaccess so they don't interfere.
rename all .htaccess files in the path eg: even one in C:\wamp\www if one exists