Unfortunately, I never owned a dirt bike while I lived in Vancouver, but all you need to do is get online and find motorcycle shops in the Vancouver area. You're sure to find a few dirt bike nuts lurking there. Vancouver is no different than any other place in the world ... there will be plenty of motocross, with a few riders at the local pro level. Check to see if the CMA (Canadian Motorcycle Association) has any events near Vancouver (or near Burnaby, BC or Surrey, BC is more likely, because they are inland but still near Vancouver.)
But ....... Vancouver is one very rainy place, so be prepared for mud riding.
I had the best times in Vancouver (I won't go into sordid detail here). :p
The only negative thing about Vancouver is that it is the San Francisco of Canada. :whoa: