Consider all factors
Hmmm, Tricky.
Cannot say I know VN that well - but I have worked there 6 times in the last year. I also used to work for a rather larger German company. So here is my two pennyworth.
Plusses - You only live once and life is not a rehearsal - so why not ?
Your Wife may really appreciate you making the effort to live in her Country if only for a while.
It would be a great experience, I live in MY and whilst the money is crap it is fun.
Negatives
Hardly any VN speak English - I doubt very much if they speak Deutsch. How good is your Vietnamese ? If it is no good then you could find life a tad lonely in terms of non-family social life.
Money, Health Insurance etc. Yes not as good - but what do you want money for ? Usually to help provide for a good woman and live a fun life.
Everyone in South East Asia is out to rip off rich Westerners at every turn. But you get used to this and if you have any wits about you you can avoid the worst. (a VN wife/family will help)
My guess is that the Grass is always greener on the other side and that after two years you will want to change again. But why not ? ?
Just do whatever you can to keep your options open, extended leave, promise of new job in 48 months etc etc. As we say in Scotland - Don't Ask - Don't Get.
Go for it !