While doing entry we can see default rules or generic rules etc for discount, how and where to apply those
Try doing a search and reading some documentation :)
This is one way -
http://docs.virtuemart.net/manual/general-concepts/207-categories.html (http://docs.virtuemart.net/manual/general-concepts/207-categories.html)
If this is not for your try searching this forum or using a general web search.