We will be in Vancouver for 2 days prior to a cruise. Should we change our U.S. dollars for Canadian $ or will our U.S. $ be accepted?

It will be accepted but I would change it - it's faster in stores and so on to use Canuck currency - some won't even give you the difference.

I am not sure what you mean by "changing money." Do you mean that you would walk into a bank and hand them greenbacks to convert to Canadian currency?

If so, that is hardly the way to do it because the exchange will cost you. If you have an aTM card issued by a bank that does not rip you off with high off net transaction fees the best way is to take out Canadian currency from the aTM.

If you are unfortunate enough to have a Bank of America ATM then you are no worse off than to swap greenbacks for Canadian currency in the bank itself. B of A charges a flat fee of $5.00 for each transaction off net plus 3%.

NOW THERE IS AN EXCEPTION. If you bank has a correspondent bank in Canada, then the charge should be zero. For example, B of A is in alliance with Scotia Bank. If I go there, my transactions are "on net."

I also carry a card based on a big mutual fund money market account that rescinds ATM fees. I have used that card extensively for currency from my money market account.

I also use a Capital One credit card which, to date, has not charged me a fee for using it out of the country.

You will find merchants who will accept your US currency, but the exchange they give won't be that great.
BUT personally I feel it is rude to visit a foreign country and not use the currency of that country. Do you think American businesses would be happy if visitors presented Euros, or Canadian dollars, or pound sterling?
Just use an ATM to get Canadian dollars.

If you get stuck, most of the Hotels do currency exchange at the desk...

Exchange rates by merchants for Can/US currency really vary. Traditionally best merchant rates are at BC Liquor stores and McDonalds!

Use he ATMs you get better rates.