Does a VOA for Vietnam qualify as the required visa for a third country??

I am a US citizen that will be flying from the US to Shanghai to Vietnam and would like to take advantage of the 72 (or I guess 144) hour free transit in Shanghai. I have a VOA letter for Vietnam (Ho Chi Minh City) which is enough for the airline, but I am nervous it is not enough for the officer at the Chinese immigration counter. Any advice would be greatly appreciated.