Entry and Exit Requirements

Hello,

I am an american citizen planning to travel through mainland China for 3 weeks, after which I will cross into Hong Kong via Shenzhen using public transit, and then fly from Hong Kong to Vietnam. The visa requirements for an American citizen specify that confirmation of an exit ticket is necessary, as I am exiting the mainland to Hong Kong via train, not air, can I submit my plane ticket from HK to Vietnam as my exit ticket?

Thank you in advance for your response!