Wow, you guys surely dont make plans that sound too easy now do you....!!
Sure you have toured Thailand, Malaysia, Cambodia & Laos to the point of boredom already??? I mean I´ve never ridden in China, in Burma, or in Vietnam (I´ve tried several times to enter Vietnam with my own vehicle, though) but I´ve had the time of my life riding those mentioned countries. They are absolutely great! Not sure, if banging my head against the bureaucratic wall to gain entrance to those tougher ones would be my idea of a perfect ´holiday´, if compared to what is on offer more easily!
But I´m not saying it is 100% impossible (well, Burma actually is nearly so) if you´re really, really determined, and well off financially - you´ll just need to consider, if it´ll be worth the time & effort spent.