The Vietnam war lasted from 1959 - April 30, 1975. The US got involved because of their Containment policy. The US was trying to do all it could to stop the spread of Communism.
The US was fighting a group of nationalists that were trying to take over Vietnam as a Communist government.
Many Americans were conflicted about the US fighting in this war because it did not directly have to do with the country, and yet it was American soldiers dying in the war.
Although the US was not directly in conflict with the Soviet Union, the US was still openly opposing their style of government by fighting a war to prevent the spread of it.