The U.S. and Vietnam share common strategic interests as we now have a common potential future threat: China.
Vietnam is now semi-capitalist and semi-free-market just as China is now and just as Germany was in 1939. It is still authoritarian just as China is now and just as Germany was in 1939.
Communist authoritarian propaganda alway stressed the "capitalist" aspect of why they opposed us but what really made Communism our enemy was their desire to forcibly impose their authoritarian ideology on others.
If Canada voted itself into being a purely Communist economy (while respecting the property of U.S. investors) , and respected civil liberties, could vote itself out of Communism whenever Canadian citizens wished and did not try to export their ideology by force of arms, the U.S. would have no problem with such a Communist neighbor as the Canadians would only be hurting themselves of their own free will.
If China had remained a purely Communist economic basket case, it would now be a far less dangerous future threat to the U.S. than the militaristic, nationalistic, authoritarian, semi-capitalistic economic powerhouse that China has become after discarding Communist economic dogma.
Communists always use dogma to suite their needs at the time. One example was that during WW II, Stalin never asked his people to fight Germany to preserve COMMUNISM; it was always fight for the MOTHER LAND.