https://en.wikipedia.org/wiki/Detention_of_Michael_Spavor_an...

China absolutely could do something if they wanted to. No treaty just means they aren't obligated to "do something", not that they can't. For example, if the US wanted to reinstate the $800 no tariff policy in exchange for Vernon, I guarantee you they'd arrest him and turn him over.

I can't find any reference to Vernon's citizenship, ie if he is a Chinese citizen or not. Which would change the math.