The usual story here (Vietnam) is that the victims are often promised a job with high earning prospect in Cambodia, or they are in debt after playing in casinos over there. Since Vietnam economy isn't in a good shape, there is no lack of potential victims. They don't really get killed either as it will cost the kidnappers, they will just get sold to another scam center. Oh and the victim can buy their freedom back if they scam enough people (my friend lost 10k USD in one such scam), not that there is many people who can do that though.