is visa needed on the vessel

Two of our service engineers should embark on the vessel Sapphire Princess in Hong Kong in November. The vessel will go to Singapore. During the trip they are expected to take part in a on job trainig. The vessel during the trip will stop in some ports in Vietnam:
Cai Lan, Vietnam
Chan May, Vietnam
Then it towards to Thailand and Singapore, where they should disembark.
They will not join the vessel as crew members but they will not leave the vessel in any port in Vietnam. They have seaman’s book. Do they need any type of visa in this case and if yes – what kind of visa they should apply.