Hi Annie
I'm sure you will have read this before if you've read the forums much but anyway.....
6 weeks seems like a lot, but be wary of trying to cover too much. China is huge and each region is a bit like a big country in its own right. For example, I went to Northern Yunnan a couple of years ago, and 3 weeks wasn't enough for me to see everything I wanted to just in that one half a province!
I would recommend starting off in Beijing, and then either travelling down the East to Shanghai, Hong Kong, Guilin, Yangshuo, Nanning and then on to Vietnam, or travelling down the middle of the country to Kunming via Chengdu and seeing Yunnan (which is fantastic, I can vouch for that) before travelling on into Vietnam.
If you're determined to see everything on your list, you'll probably need to take at least one flight. Probably the best bet would be to travel to Hong Kong via Shanghai, then fly to Chengdu, see a little bit of Szichuan, on to Kunming, see a little bit of Yunnan (e.g. Tiger Leaping Gorge) , get the train to Guilin and then on to Vietnam. I wouldn't try to fit in more than that!!
Rob