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I think I know the answer. If a Chinese passport holder crosses into Hong Kong, they can use Visa on arrival service at the Saigon airport, right?
As a Chinese passport holder, you can use Visa on arrival to Vietnam regardless of where you are.
Please keep in mind that, to use Vietnam visa on arrival, you need to apply online for a Visa approval letter first. Without it, you cannot get on the flight to Vietnam.
Here is the link to apply: https://booking.vietnam-visa.com/step1.html.
Should you need any further information, please feel free to contact us.