China visa requirements will hopefully have improved by Jan next year. If not, you will be likely using an agent in HCMC or Bangkok to get a visa (with faked flight and accommodation provided by the agent).

You will need to get a Vietnam visa in advance (in Bangkok if that is your entry city) while Laos you can get at the border.

I think there is enough to do in those countries without visiting Japan (and going there would mean a big increase in budget). Also, are you considering Cambodia as well?

I would advise perhaps staying clear of China till at least end-February, especially if you want to travel by train and long-distance bus. Otherwise, you'll get caught up in the Spring Festival (Chinese New Year) transport chaos, which would be yucky, especially for a 1st-timer to China.

Vietnam also celebrates Tet (Vietnamese New Year) on/around the same dates, but I have no idea how it affects travel and transport.

Japan doesn't have the same festival, so I presume you'll be ok there? Not sure about Thailand.

Once you figure out how long and when you'll be in China, let me know if you want help fixing up an itinerary. You could email me at ellyse99@yahoo.com -- put "TripAdvisor" and your own username (eg agentx) in the subject line, thanks.