Pretty much all of China lies between the Vietnam border and Beijing. You could take a western route, via Yunnan and Sichuan (spectacular scenery, pandas, and really great- hot- food) then on to Xian (ancient capital of China, with terra cotta warriors), and finally Beijing.
Or you could take an eastern coastal route, via Hong Kong and Macau, Fuzhou and Hangzhou to Shanghai (this is the more modern and prosperous China). Then continue north to Qingdao (beer capital of China, and a nice place to chill out for awhile), then Beijing.
I just wouldn't go the middle route, which is "mei yi si" (uninteresting). Well, there are some places of interest, but you'd miss all the major sights.
China is a big country. You won't see everything in a month, especially if you are travelling by train, instead of flying.
Bon Voyage!