I accidentally hit your questions!
I am living in Shenzhen in China and hope to help you guys out with your visa (-questions)!
1. The reason you are being rejected for a visa in Vietnam, is that officially China has introduced the rule that we may not apply for a Chinese visa any more from a foreign country. However, lots of people apply for a visa in Hong Kong.
2. The schedule you have to provide the Chinese government for visa; who says you have to commit to that one?? If I were you guys, use your computer skills, make bookings, delete them again, book flight tickets, delete again, and there you are. China does not check them, it is just that they have the formal idea of where you are. However, if you can present them anything and then delete it all again, you are still able to travel freely!
3. If you can't get the visa in Vietnam or wherever you are; travel to Hong Kong. Maybe over budget and unexpected, but I think it is the best way if you really want to travel through China. In Hong Kong you may go to the Commissioner's Office (http://www.fmcoprc.gov.hk/eng/xwfw/gszxxw/t426104.htm) or China Travel Service. They can help you getting your visa. From Hong Kong there are many trains going into China.
4. Train travelling is still allowed into China, only not for foreigners. They don't want any foreigners to enter the country over land, they prefer you come by air. However, I have still been travelling between hong kong and China over land, so as you all know; Hong Kong is different as it is meeting with the Special Economic Zone; Shenzhen.
5. If you want to visit Tibet; forget it. I wanted it too myself, but no permits are given and there is a widespread silence concerning Tibet. I do not expect Tibet to open up before september.
Check the above site of the Commissioner's Office in Hong Kong, information is on there. If you want more info from people who have lots of experiences with Chinese visa, check www.geoexpat.com and search for 'China visa banned'. This thread is 50 pages long and if you quickly look over it there are many answers to find there.
Any more questions, just ask! I have lots of information on the Chinese visa and I will apply for one again next week...
I have been travelling on F (business) visa until now, but as it has become more strict to get a business one too, it is maybe needed to get a tourist one for me too....
Hope you guys still have a great trip!!