As per this channel, a relatively famous, well-informed and objective Youtube outlet about Vietnamese defense issues, the Vietnamese oil tanker was secretly helping Iran to ship oil to an East Asian market, probably in violation of some US embargo.
The oil tanker, owned by a little known company headquartered in Hanoi, Vietnam, was doing its routine work. Then the US navy ships appeared, trying to seize the tanker, but Iranian navy had successfully deterred the action by US navy
I am not sure if it is a 100% correct story, but there is little information from Vietnamese official media. A sensitive issue to all Iran, Vietnam and the US, I suppose.