why vietnam? if you're heading north singapore-malaysia-thailand, vietnam will add quite a bit of mileage. you would go straight thru laos to kunming.

china may cause you some difficulty.

i believe the maximum duration of stay with a tourist visa is 30 days.
i know of some people that have camped in china, although i never have. almost always possible to find a hotel with rooms under $15. out in the boonies you can find basic lodging at some gas stations or truck stops.
assuming your phone works with chinese frequencies, will you be using a chinese SIM card? for foreigners, that usually requires registering with passport at the main telecoms office in whichever town.
facebook and google won't work without a VPN. you'll need to install the VPN software beforehand, as getting one set up in china can be problematic.
learn to use baidu maps for when your googles won't work.
for communications, you can install chinese weixin (wechat).
wifi is everywhere in cities.
internet bars are restricted, require chinese ID card to register and use internet.
in towns, you can ask for a "computer room" at many hotels. costs an extra $1-2, comes with desktop computer with internet.