Yes, it's not as if this is some secret that nobody knows about. It is extremely well-known that this trade goes on. The police are aware of it as well. I would be willing to bet that they even know where the "farms" are but don't do anything about it. I would like for some sort of global animal-welfare organization to pressure the government to do something about this.

At the moment the only illegal part of dog meat trade in Vietnam is transporting them across province lines. Since these lines are not patrolled, this is unenforcable and I don't believe that they ever really had any intention of enforcing it. Some animal-loving politician probably got in the news for their compassion and then they just carried on doing what they always have.