The DSA has a group that visits Cuba, so perhaps you could go really hard on activism to get into it. Other leftists groups probably also have their own delegations for things like that, though it is unlikely you'll get to China that way afaik. If you're trying to get into China, I think your best hope is through (non-socialist) Chinese cultural enthusiast groups, though I'm just speculating and for all I know Friends of Socialist China has a delegation.

There are probably groups that visit Vietnam, so perhaps if your friend can go to China easily, they could use the rail system to go down near Vietnam and take a short flight in?

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There's no need for a tour group for China, Vietnam, and Laos. Clueless college grads go backpacking in Vietnam and Laos all the time without issue. China can be a little tricky without knowledge of Mandarin but if you spend a month or so learning travel phrases you'll be fine. You will need a tour group for the DPRK (can't go if you're a US citizen) and Cuba if you're a US citizen (US citizens can only go for "support of the Cuban People").

Flights to mainland China from the US and many other cringe countries are super expensive RN, but you can save on costs by flying into Hong Kong, Taiwan, or south Korea first and booking a separate flight to mainland China.