Vietnam travel visa

We are arriving jan9th, 2019 at HCM airport staying in hotel 1 night then a flight to Siem Reap, Cambodia . We then take a cruise returning to Ho Chi Minh Jan 17th for a return flight . AS we are entering Vietnam once By air on Jan 9th and once by seaport jan 17th do we require a single travel visa or a 2nd one for re entry on 17th
thank you
Dennis