But a decent number of them appear to switch off most/all validation once you select a non-US address, which I think is a perfectly acceptable thing to do. (That's if they even support a non-US address, that is.) I've never seen any data but I wonder how much help validating addresses even is.

The "must have a postal code" is by far the biggest problem. I can't recall a website that actually can cope with not having a postal code.

In practice you can put in any number and the local postal delivery here will ignore it/figure it out since it won't be the first time a foreigner has put the wrong address on something in Vietnam.