There are lots of vet bringback SKSs and pistols available in the US. Reason is very simple: Americans did not have to put up with our anal restrictions.
Bases where returning men were being readied to come home had special facilities for papering weapons coming back into Zone Interior with the troopies. They would paper anything which was still legal in the US, with the exception of full-autos. There is a famous photo of a line of guys waiting to have their bringbacks papered, big sign saying "SKS? OK! AK? NO WAY!" Enough SKS rifles came back to cause a 40-year shortage of brass for the Italian Carcano; Carcano brass (once very common, including US-made ammo) was bought up and converted to 7.62x39 to feed all the SKSs. Result was that Carcanos ended up selling, in some places, for half the price of a single box of Norma ammo.
SOME Viet-Nam bringbacks DID come to Canada. There is one in the CFB Shilo Museum, an AK which was given (fully operational) to a Canadian Major who was over there. The Major brought the thing to Canada and gave it to the Shilo Museum. The RCMP turned up later, insisting that a Canadian Army Museum had no right to possess weapons on a Canadian Army base, forced the Museum to register the thing, then seized it and welded its guts up before returning it. I know about it because they didn't have a manual for the critter and it was a popular exhibit, so I wrote them a manual for it.
So they do exist. It's just that, in this country, getting your hands on one with papers can be awfully hard.
If the Canadian Militia had been allowed to volunteer (as was rumoured several times and nixed by Ottawa) to serve in the Nam, doubtless they would be much more common.
Helluvva thing, though, when there's a war going on and your friends are on one side and your Government is on the other! Canada was turned into a Fifth Column area for the Fourth and Fifth Internationalists, the delightful folks who organised all those "spontaneous" protests against the Evil Wall Street Imperialists And All Their Running Dogs And Lackeys..... same folks, come to think on it, who are running most of the "environmental awareness" movements today, screaming for us to shut down one industry after another and never, ever saying a naughty word about the biggest polluter in the world: their dear buddy Communist China.