你的美国签证过期,对你回中国这一程的飞行没有影响。你是需要办理过境签证。没有的话,航空公司可以不让你上飞温哥华的飞机。那些在加拿大机场办了过境签证的是加拿大边境人道主义,原本是航空公司不能让这些乘客上飞机的,但既然人已经到了加拿大了,就给他们发个签证打发他们走德了。
温哥华转机是要出关然后拿行李的,所以是需要的。但是再机场是可以办理的。下来后要经过助剂厂的移民办公室,你要拿着证件,机票给移民管审查,他回稳你一些问题,但是一般很快就会办下来。
Usually, you can apply Transit visa at Canada airport. But if you don't have enough layover time ( more than 2 hours ) for next flight, this is not a good choice.
Your US Visa has expired, so this is really not a good sign to apply Transit visa at Canada airport.
I suggest you to purchase the air ticket not going through Canada. You might only need to pay tra canceling fee, but it is still better than being stuck in Canada and miss the next flight.