If I apply for Vietnamese nationality, do I have to renounce my American nationality?

Pursuant to Clause 3, Article 19 of the 2008 Law on Vietnamese nationality:

3. Persons naturalized in Vietnam shall renounce their foreign nationality, except for those defined in Clause 2 of this Article in special cases, if so permitted by the President.

Therefore, you will renounce your U.S. nationality in principle. However, except in the case of Clause 2:

2. Those who apply for Vietnamese nationality may be permitted for naturalization in Vietnam without having to fully meet the conditions prescribed at Points c, d and e, Clause 1 of this Article if they fall into one of the following cases:

a/ Being spouses, natural parents or natural offsprings of Vietnamese citizens;

b/ Having made meritorious contributions to Vietnam’s national construction and defense;

c/ Being helpful to the State of the Socialist Republic of Vietnam (Clause 2, Article 19 of the Law on Vietnamese Nationality).

Therefore, not all cases of applications for Vietnamese nationality has to renounce your American nationality so you can base on one of the above conditions to know exactly.

Best Regards!