I don't know, either. Bob's our resident expert on Vietnam ; I'm sure he'd know.
A Vietnamese girl who I met on a Halong Bay cruise - she was with her English boyfriend - told me it was not customary to tip.
I give tips to hotel staff, regardless. They are underpaid and overworked, imho. And under-appreciated, I think. A tip at the beginning of the stay ensures excellent service, which in turn earns them another tip when I leave.
Winners all around!
In Thailand I tend to stay at the same hotels and kind of know the staff, so I tip them at the end of the trip.