We are US citizens and were visiting Canada. Flying home to USA from Vancouver we went through passport control when departing Canada, but did not go through passport control or customs in the US. How is this possible? Even with a passport and customs check in Canada, wouldn’t USA still require control at its border?
You experienced USA Preclearance, where US Immigration and US Customs agents are deployed in some foreign countries; currently, fifteen non-US airports have US Preclearance facilities. You can read more about it on the US Customs and Border Protection Preclearance webpage.
US-bound flights with passengers who've cleared CBP preclearance are essentially "admitted" to the US before boarding the aircraft. Thus, flights arriving in the US from preclearance facilities are treated at US airports the same as flights originating within the US, and can deplane passengers and their luggage without further CBP examination or formalities.