The US customs is 150 yards within US border. The Canadian customs is 100 yards within Canadian border. In between there is a Peace Arch park. There are not gates when you either leave US or Canada. The road is one-way for each direction. If you accidentally leave US by car, you have to go through both Canadian Customs and US customs to come back to US. If you can't get in Canada, you have to go into Canadian border and turn back to go through US customs.
There are three border entrances in Vancouver area. You may try the other two.