Eleven years ago, I was travelling to my country with layover at Vancouver. There, I was questioned because I did not have transit visa. This year, I am planning to visit some friends in Canada. I am a permanent resident of the US. Will that earlier event of questioning in Vancouver affect my entry now as visitor of Canada? Will I be allowed to enter?
US permanent residents are normally allowed to visit Canada without a visa (and even without a passport!). You will need to carry your green card with you and show it to immigration officers. But since you have had previous history with Canada, you may expect to be asked about it when you arrive. You should answer such questions truthfully. If you were not refused entry on your previous transit, you should not have any serious problem now.