According to the article number 20 of 47/2014/ QH2013 circulation about the Conditions for Entry Vietnam Immigration issued on 16th of June 2014 with effect from 1st January 2015: A foreigner shall be granted entry into Vietnam with unilateral visa-free entry when the foreigner have a passport that is still valid for at least 06 months, and the entry date must be at least 30 days from the previous exit.

Currently, Vietnam's unilateral visa exemption is applying for citizens of 13 countries, including: Korea, Japan, Russia, Denmark, Sweden, Finland and Norway, Belarus, Italy, Germany, Britain, France and Spain.

Citizens of those countries can enter Vietnam without visa within 15 days. However, they want to enter into Vietnam under unilateral visa exemption in the next time have to wait at least 30 days from the last day exited Vietnam.
 
For example: A German citizen entering Vietnam dated 1st July 2015 under unilateral visa exemption and leaves Vietnam on 10th July 2015. If he wants immigration back to Vietnam, he has to wait at least 30 days (it means after 10th August 2015)  to enter Vietnam under the unilateral visa exemption.

If you want to enter before this time, please apply for Vietnam visa.

 --------------------------------

Article 20 (Conditions for Entry Vietnam Immigration): Conditions for entry

A foreigner shall be granted entry into Vietnam when all of the conditions below are satisfied:

1. The foreigner has a passport/laissez-passer and a visa.

Any foreigner that enters Vietnam and is unilaterally granted visa-free entry must have a passport that is still valid for at least 06 months, and the entry date must be at least 30 days from the previous exit;

2. The foreigner is not suspended from entry as prescribed in Article 21 of this Law.