Entering Vietnam and China without proof of onward travel?

I’m planning to take the train from Hanoi into Nanning China and then take the train through China into Mongolia from Beijing. However neither of these tickets are available in advance (Vietnam to China one is walk up only and the Beijing to Mongolia one is only released the 30 days beforehand.

What are the chances I’ll be allowed into the countries without proof of onward travel? Should I just buy some refundable flight tickets?