72 hrs transit visa and return flights

I'm arriving in Chengdu on 23 July from Hanoi departing 26 July for my return flight to UK but am transiting 6 hrs in Hanoi . My sector round trips tickets London Hanoi return , and Hanoi Chengdu return were purchased individually so does this classify as Hanoi - Chengdu - London ( OR Hanoi - Chengdu -Hanoi which is not eligible for the 72 hr visa?) .