As for citizens of Puerto Rico, an ordinary Puerto Rican passport holder cannot enter Vietnam without a Visa, i.e. citizens of Puerto Rico are required to apply for a visa for entering Vietnam, except for those who are married to a Vietnamese visa holder. Also all Puerto Rican nationals are required to be guaranteed for, by an agent from Vietnam. 

Which countries are allowed to enter Vietnam exempt of visa?

Citizens of the below mentioned countries are allowed to enter Vietnam without needing a visa for a fixed amount of days. They are:

Brunei and Myanmar: 14 days 

Being a Puerto Rican citizen, do I need a Vietnam visa?

To answer the question, there are a total of 17 countries, the citizens of which do not require a visa for entering Vietnam and staying there for a limited amount of time within a 30 days cycle. The 17 countries which allow visa free entry for its citizens to Vietnam are: Denmark, Finland, Japan, Norway, Russia, Sweden and South Korea are allowed to enter Vietnam exempt of visa requirements for a period of 15 days in a 30 days cycle. 

Citizens from Cambodia, Indonesia, Laos, Malaysia, Singapore and Thailand can also enter Vietnam free of visa and stay for 30 days. And the citizens of Philippines can come to Vietnam and enjoy a visa free stay for a period of 21 days. Finally, nationals of Brunei and Myanmar are allowed a visa free stay in Vietnam for a period of 14 days in a 30 days cycle. Note, that by a 30 days cycle, we mean that a person can only stay for the aforementioned time period allotted by the Immigration department of Vietnam by the Immigration law and are required to leave Vietnam for at least 30 days before they can come back again. In case they exceed their free of visa staying period, they will be required to apply for a visa and also if the citizens of the above mentioned countries return to Vietnam in less than 30 days break, they will also require a visa.