My nationality is exempted from Vietnam Visa for 15 days. Do I have to wait 30 days from the time of departure to reenter Vietnam on a unilateral visa exemption if I already did so the last time?

The Law amending and supplementing a number of articles of the Law on entry to, exit from, transit through and residence of foreigners in Vietnam in 2019 (effective from July 1st, 2020) has abolished the regulation that the interval between the entry date and the previous exit date must be at least 30 days.

Therefore, you don’t need to wait until 30 days to reenter Vietnam on Visa exemption.