Dien,
In my experience: stumbleupon.com has the higest potential to generate traffic - if you're not already extremely famous.
Reddit.com is ok. Digg.com is good only if you can make it to their front page (which would require *atleast* 50-100 diggs from your community fairly quickly.) But with stumbleupon.com - you can get a lot of traffic with just a handful of votes. So do check them out.
This may help: Its a plugin for vbulletin:
http://www.vbulletin.org/forum/showt...eupon&page =5