Sub Domain URL structure

I have seen lots of vB sites and some configure their directories different than others,

For example vbulletinsetup.com URL is

'http://forum.vbulletinsetup.com'

and my forum index page is

'http://www.crimecasefiles.com/forum/'

Could someone explain to me why you would set it up differently,

for better seo?

and I take it that you would have to set up your forum structure differently that how I would set mine up?

I am assuming that there are reasons that would decide why?.
I have always wondered why, and thought I should find out.

Thanks

Setting your site up depends on what is in your site and what it's about. As you don't even have a homepage (just redirects to forum), I see zero point of being installed in a folder at all. I would migrate your site to root.

vbsetup runs a blog in their homepage and chose to run a subdomain instead of a subfolder. I personally do not agree with their setup.

Generally, I suggest keeping everything that is your CORE website in root (in most our cases, vb). Especially with vb4 and the cms, many people have regretted being installed in a folder.
Use subdomains when the 'product' is a complete seperate offering from the main .com
use a subfolder when the subfolder supports the main .com

Using google as an example...
all their search related stuff is in google.com
help files are on google.com/help and so on, as they support google.com
all the maps, which have nothing to to with search (more or less) are on maps.google.com
email the same way, mail.google.com


The only time i would suggest using a subdomain on a similar topic is when you need to use the power of subdomains and DNS to point it to a different server/etc than the main .com. You can't do that with a folder.