Vietnam - Shanghai - Vietnam and want to apply 72-hour free transit?

Hello,
I am Germany citizen, i would like to ask, if i flight from Viet Nam to Shanghai in 48 hours and come back to Viet Nam, 72 hour free transit policy can apply this case
Or I need to flight to third country, and in this case : VietNam -Shanghai- Hong Kong -Viet Nam can accept.
Looking forward to hearing from you soon.

Many thanks & best regards
Mechaela