I went to Vietnam a long time ago (ten years) and HK about 4 years ago. I'd definitely go to hong kong with kids over Vietnam. HK is so clean and organized and really fun! If it's just for a stop over, what about Kuala Lumpur? We loved it with our two, and we stayed near a massive park with wading pool. Also the Malaysian people would have to be the nicest people on earth! Equal to Thai people I think !

Vietnam is exciting, but it's busy and dirty and is so similar to Thailand in many ways.