Subdomain or Subdirectory for vBulletin forum?

[محروم.كوم]

Hi guys*
I've been using sub-domain (http:// forum.myseite.com) for my site for about a year now* and everything is fine till I used Google site search and I was surprised about the results.
When I searched for the main domain (site:http://www.mysite.com) I only got 7 results* and all sub-domains links are ignored* but when I search for the sub-domain (site:http://forum.mysite.com) I got thousands of links (almost all links of my forum). That makes me wonder:
Which on do you think is better for vBulletin forum regarding SEO and other stuff? sub-domain http:// forum.myseite.com or sub-directory http:// www.myseite.com/forum __DEFINE_LIKE_SHARE__