Lived out there for a 4 years or so.
Never went to Vietnam.
So much to see in Thailand if you can be bothered to get off the tourist route. Hiring a car is a great way to go. It’s not dangerous if you keep your head and realise that if a truck is pulling out, then it’s pulling out and isn’t going to stop.
Laos is a nice spot to visit as well. Mrs Q and I used to take our well earned breaks there.
However if you want something completely different you could go peninsular malaysia – Cameron Highlands, Perenthian Islands, Tama Negara National Park, Cherating Beech resort, Kuala Lumpa and a couple of places south of KL that I’ve forgotten.
It’s all within easy traveling time as well.