Could you elaborate on your question? I assume you're referring to the Vietnam War, but Vietnam was divided in two separate nations (North and South Vietnam). Are you asking why the United States couldn't defeat North Vietnam and the PLAF? Or are you asking why the United States had issues dealing with the South Vietnamese government?

Follow up: Why it couldn't beat North Vietnam? in Korea they were stopped by China, but in Vietnam, there was less opposition so, why didn't they reach the same advantage that they reached in Korea?