144h while arrival and return ?

I am planning the trip with two independent return flights, so
Germany-Shanghai-Germany
and
Shanghai-Vietnam-Shanghai.
If I understand this right I can use 144 .. 164h in after arrival in Shanghai til departure to Vietnam.
Can I use this after return from Vietnam, so before flight back to Germany a second time?
Don't want to stay so long (1-2 days) , but without a buffer in Shanghai I am afraid to miss my flight back in case of bigger delay ...

Thank you