Vietnamese passport holder/ transit in Beijing (for 4 hrs) & in Shenzhen (for around 20 hrs)

Hello,
I have a flight from Frankfurt to Hanoi, and transit in Beijing for around 4 hours and a flight back from Hanoi to Frankfurt and transit in Shenzhen for around 19 hours 20 minutes. As far as I know, I am eligible for 24h-free-visa in those airports?
Also, if yes, am I only allowed to stay at the airport(s) during those transit time, or can I go through customs to visit the city listed (e.g. sight seeing/shopping) (Beijing or Shenzhen) and come back the airports to catch my next flights with the 24 TWOV without any problem?