Communism and socialism do not automatically shield you from exploitation. In theory it is supposed to protect you from an individual or corporation capitalizing your excess production for profit.

Vietnam is of course a mixed economy and thus still has similar pitfalls as any market economy. Though they do exert more control and protections on certain state enterprises like energy and banking. They also still own the land that corporations lease from.

So in short, the answer to your question is that exploitation is still happening because of capitalism.