You can use American money and you will not get a second look. US money is not new to us and we get it all the time especially places like Vancouver which are import cities. The exception to this would be, as mentioned above, some public transportation. ANY store or restaurant will take US funds. You may not get the best exchange however if your only using a few dollars cash and mostly using your credit card, you will not break the bank. Even our vending machines take credit cards. Bring lots of smaller US bills and you can use them for tipping, again the value is more to us so we are very happy to get it. Myself, I seldom have any cash on me at any given time, not so much as a Loonie. I use my credit card for everything. That's how cashless Canada is becoming.
Also, keep in mind, again as mentioned above, we have no bills smaller than the $5. Our $1's and $2's are coins and if you get those and take them home you may have a hard time exchanging them back to US. Banks usually only want rolls of coins, they will not deal in one or two and some US banks will not deal with them even then.
Don't bog yourself down with to much Canadian if your only going to be in Vancouver for a few days, your credit card will work just fine along with your US. Just make sure your bank understands you will be traveling so they don't put a hold on it. Have a great time!!