I have lived in Vancouver for a year now and have explored the road cycling world here. I can't say anything about the mountain biking though.
If you are planning to ride on the Gulf Island, I strongly suggest Salt Spring Island. You can get a 100 km ride there, trying just about every road. Make sure you have a map and plan your trip accordingly. You can also ride from Vancouver Island to the Brentwood Bay - Mill Bay Ferry, ride from there to another ferry (can't remember the name at the moment) to Salt Spring Island Vesuvius Bay, then from Vesuvius Bay to the other ferry station (again my memory fails me) at the other end of the island back to Vancouver Island (Swartz Bay). If you intend on going to Victoria anyway, that can be a good plan.
I also have written a detailed ride around the Saanich Peninsula (where Victoria is situated on Vancouver Island) which I could send your way if you want.
Around Vancouver itself there is also a lot of good rides, long and short, to do on the road. If you want some group action, join the Dizzy Chicks on Saturday morning, 9:30. (I'm a member. See our web site: http://www.dizzychicks.com for the details of ride meeting points.) There is also a weekly ride (Sunday morning) east of Vancouver that's just as exciting. I have ridden less in the distant perimeter of Vancouver because I don't have a car so all my rides start from home. I could inquire for you if you want.
I hope you enjoy your trip! Let me know if you need more info!