Yes, US citizens are required to obtain a visa to enter Vietnam. There are several types of visas available, and in order to know which one you need, you have to know when you’re going, what you plan to do, and for how long you plan to stay in Vietnam.
Regardless of your status or desired length of stay in Vietnam, you should consult with the Vietnamese Embassy or the local consulate nearest to you to confirm which type of visa you will need.