According to announcement of the Vietnam Immigration Department following the successful agreement between US Government and the Vietnam counterpart, all US citizens to Vietnam now MUST apply for 1-YEAR MULTIPLE ENTRY VISA regardless of how long they are in Vietnam and their purpose of visit.
This policy took effect from August 30, 2016.
With its entry into force, other types of 1-month single/multiple entry, 3-month single/multiple entry or 6-month multiple entry visa are no longer available for US passport holders then.
This policy also means that all US citizens who use Vietnam Visa Online (also known as visa on arrival) must pay US$135 as stamping fee at Vietnam airport to get visa stamped. This is the amount applied to 1-year multiple entry visa.
Its announcement really raise a shock among US travelers, especially those who wish to stay in Vietnam for a short time, but it is unavoidable.
For more information, one can contact Vietnam-visa.com which is considered an expert in this field with over 10 years assisting travelers to obtain a visa for Vietnam.