Just returned from 2 weeks in Vietnam. Obviously didn't do everything. But did Hanoi, Halong bay Ninh Binh, Hue Danang and Hoi An.
For families it was so much easier to get good hotels for 3 people or more, no 800bht extra bed charges. Hotels were excellent and like for like half the price. Everything in Vietnam is on time. Trains buses tours, taxis etc.
I found pollution a little worse for sure and I much preferred Thai food generally. Ban me sandwiches were nice though. Nightlife in Hanoi similar to Bangkok, I didn't see a red light area but perhaps it's there but I wasn't looking.
Halong bay nicer than Phang Na bay area. Can be crowded though. Ninh Binh loved it, nicer than anywhere I have visited in Thailand. Cycled everywhere there it felt so safe to do so.
Hue a lovely city buzzing with life at night preferred it to Chiang Mai. Danang is nicer and cleaner than Pattaya, especially the beach and sea front area. If there is a red light area I didn't see it.
Hoi An a wonderful place in the evening, with the boats and lanterns. Like loy kratong every night but with boats in a lovely old town.
More bargaining required in Vietnam and more hard sell than Thailand. Apart from food Vietnam for me is nicer especially for a holiday.