Vietnam has no public bathing culture. Even at home, showers are the norm. So for a Vietnamese visitor, a Japanese onsen or sentō is unfamiliar before any rule is mentioned — starting with the premise itself: undressing completely and sharing a tub with strangers.
Said in advance, the points below save everyone discomfort on a company trip.
Wash before you get in
The most important rule, and the least often explained. The tub is not where you clean yourself. It is where you warm up once you are already clean.
Sit at the washing station, wash your hair and body, rinse off every trace of soap, then get in. Some people will say a quick rinse is enough; on a first visit, wash properly.
The towel stays out of the water
You carry a small towel in with you, but it never goes in the tub. Fold it on your head, or leave it on the rim.
The large bath towel stays in the changing room.
No swimwear
Both onsen and sentō are bathed in nude. Almost no facility permits swimwear. Without warning, someone will pack a swimsuit.
Tie long hair up
Keep hair out of the water. Bringing a few spare ties for anyone who forgot is a kindness.
Check the tattoo policy first
Many facilities refuse entry to anyone with a tattoo. Some allow small ones if covered with a patch, and a minority advertise themselves as tattoo-friendly, but these are exceptions.
If someone on the trip has a tattoo, being turned away at the door is an uncomfortable moment for everyone. Check the venue's policy as soon as the venue is chosen.
Do not drain the tub
The water serves the next bather. When you get out, leave the plug alone.
How to pass this on
To a Japanese bather, none of this needs saying — which is exactly why nobody says it. Assume it will not be known, write it down once, and send it before the trip.