Will this visa-free transit let me back in the country?

I am a US citizen currently in China (2/13) on a 30 day single entry category M. I am interested in leaving the country and visiting Vietnam, though I have a flight out from Shanghai on 3/18. If I go to Vietnam, will I be let back into the country if I fly after 3/15 to catch my flight from Shanghai? Would I be able to leave the airport for less than 72 hours?

Thank you very much for the help.