72 hour free transit in China followed by Visa On Arrival in Vietnam

I will travel from the US through London ( just changing airports in London- not visiting) to Shanghai. I would like a 72 hour free transit in Shanghai as a tourist. I know I need an entry permit for Vietnam, the 3rd country, in order to get the 72 hour free transit for PRC. I will go to Vietnam from Shanghai and am planning to arrange for an entry permit on arrival in Hanoi. Will my approval letter for the VOA for Vietnam be sufficient for the PRC 72 hour free transit?