They wouldn't sanction Vietnam. Vietnam threaten US to join China camp if they sanction them.Well that is interesting. Vietnamese are loud mouths and if there ship full of oil is taken by iran then why r they silent?
Vietnam have been buying iranian oil secretly at cheap price and i guess there was payment issues which lead iran to seize the ship. By law, the US should sanction vietnam for not abiding by sanctions. But i guess in real world laws are not same for everyone.