However we have hit a snag because the Chinese embassy in Vietnam says we can only get a visa there if we are flying from Vietnam.

I am a UK citizen living in Vietnam. A friend and I are flying from Ho Chi Minh to Russia and then are going to do the Trans Siberian Railway - so train through Russia, then Mongolia and then China. So we don't know how to proceed. I just don't know how I can get into China to catch a flight from Beijing to Madrid I have booked. Would I be able to book a flight from Mongolia to Beijing that is less than 72 hours until my flight to Madrid? Would that be covered under the 72 hours free time? I can't work out if it has to be connecting flights, which it wouldn't be. Any help would be greatly appreciated :-D Thank you in advance.