We have a single entry visa can we also use a transit visa prior to entry?

We are backpacking around Asia and plan to visit Korea, Vietnam, China and Russia. We have a single entry L. Our flight from Hanoi to Seoul has been cancelled and our airline has provide a replacement flight where we have a 20 hour stopover in Guangzhou. They say they will arrange for transit G but we are concerned that we may then have problems with our subsequent entry into China. Can you advise please.