Probably the only thing you would need cash for is tips, pay a newspaper, maybe cabs (I've never taken a cab in Vancouver so I don't know if they take credit cards), small purchases that you don't want to use a credit card for, that kind of thing. Most stores in Vancouver will take U.S. money and give you change in Canadian. So you could purchase something and use the change for your small stuff.
If you use a credit card, you will get charged the amount in Canadian (at the shop or whatever) money but the coversion will be done by your credit card company and will show up on your bill in United States dollars. Sometimes you will get a better rate that way because vendors could round the amount up, as anyone probably would, whereas the credit card company will charge you the exact exchange rate. Probably only talking a few cents to dollars depending how much you spend though.
Credit cards work in Vancouver cabs as well as restaurants and hotels. The cabs typically have a sign asking you to inform the driver that you will be paying by credit card. They like to start filling out paperwork at red lights. They tend to give you back a carbon of your card, so I quickly scratch out my card numbers for security.
We didn't bother to get Canadian currency. Used $US for tips and for small purchases (snacks, inexpensive souvenires). Some stores (although none we encountered) give you a 1:1 exchange rate, but with amounts this small the amount you're losing in the deal is negligible.