Am I qualified for the 72 hour visa? If so, do I need to go to the police station to check in?

Hi, I have a return flight from Vietnam >Beijing>U.S.A. I'm an American citizen. However I plan to visit S.Korea then fly to Beijing to catch my flight to U.S. So I will not use my flight from Ho Chi Minh to Beijing. It will cost me more to cancel so I will just not use it and connect in Beijing from South Korea instead. I will stay in Beijing for two days.
Thank you!
Dat