Visa (and, accordingly, an invitation too) to Vietnam is not required if staying in the country does not exceed 15 days.

If there is a probability, that you will stay in Vietnam longer than 15 days, it is better to make a visa in advance at the Vietnamese Embassy. To obtain a visa, you must have an invitation, which is issued by the Migration Service of Vietnam after checking your data. Simply call at the airport and you can not paste a visa.

If you want to stay longer than 15 days - an invitation is compulsory.