I was in Vancouver for 2 months last summer and had no problems buying a day pass, faresavers and a monthly pass with (non-Canadian) credit cards, although I didn't try to buy them at the airport.
These days most retailers will no longer take credit cards for transit fare purchases. The percentage fee they have to pay for the credit card transaction is often greater than the profit margin on the fare tickets.
Cash or payment via ATM card (called "Interac" in Canada) is usually fine.
Note that this is retailers like 7-11 that I'm talking about here.
Credit card payments at ticket machines at SkyTrain stations are still accepted but, IIRC, you can't buy a daypass at YVR station.