I have been on both and for your ages, I would suggest Vietnam. China has lots of internal flights at huge airports, (think JFK plus, DFW, sized) So just getting on and off the flights is a huge trek. China also has lots more history, and it felt like more sit-down meals. Don't get me wrong China was amazing, and the hotels are ultra deluxe. Vietnam is a bit slower paced and although there are several flights they are at smaller airports and the transfers are just a lot less exhausting. In addition, there is some beach relaxing time in Vietnam. Angkor Watt is absolutely incredible, and they have a super fun scavenger hunt there. You also get to ride elephants, go on boats, make crafts, just a lot more activity oriented stuff versus actual sightseeing.