In reply to The Grist:
Yep, I second that (on SEA at least). A serious "cultural" experience is a little hard to get in Vietnam and you'll be quickly herded on to the tourist trail. In fact you have to work harder at staying off the tourist trail - transport, accom, food, its all aimed at a certain clientele, as it is in much of the region.
I wouldn't be too concerned about not having "done" Vietnam while it is still "authentic" either. Vietnam is hugely different now to what it was 10 years ago, and is no doubt different to what it was 20-30 years ago. All these countries are changing so you are just seeing something different at different times. The modernisation isn't all bad either.
My money is always on Japan.
Yes its expensive (or not cheap). But it is a fantastic country, awesome people and a hell of an experience. I've only traveled there with my Japanese (then) girlfriend so never really met other travelers and not had to contend with some of the usual language difficulties. But there is a stark difference between the tourists you see and the tourist experience you have there and in Southeast Asia.
SEA is like going to Magaluf/Benidorm/Aya Napa. It's cheap and attracts a slightly cheap crowd - loads of kids, the Gary Glitters of the world, and generally a lot of unsvoury sorts who revel in the fact they can either misbehave and bribe their way out of trouble or live like a king and have the locals as their underlings. A tourist in Japan, on the other hand, has to (within reason) fit in to the culture, observe local custom as much as possible and generally be much better behaved. It might sound like a small thing but it actually makes a huge difference to the experience, and in terms of culture, perhaps surprisingly, it is therefore much easier to actually get closer to the "real" Japan than it is to Vietnam.
Just my opinions...