Transit-Vietnam airport

Hi all, was in transit at HCM city on a Vietnam airline flight from BKK-Fukuoka where i was in transit for nine hours. Didnt need a visa, but will be doing the reverse flight in 3 months only this time i will be in transit for a sweet 21 hours. Will not be leaving the airport, was wondering if this length of time will ae a problem ? will a Vietnam visa be required? The agent i bought the ticket from said no problem, but i do not trust any agents who are simply after commission. Cheers for amy rellevant feedback.

As long as you stay inside the aiprot, a visa is not required as, for all intents and purposes, you don not enter Vietnam.