Transit less than 24h in Guangzhou

Hello, I'm from Vietnam and gonna go back from Seoul to Vietnam in this weekend, I will transit in Guangzhou around 18 hours. In this case, can I get the stamp for 24 hour direct free transit? Last week when I contacted with China Southern Airline in VN, they seems like didnt know about transit direct less than 24 hours, they even told that I can't go out of GZ airport without transit visa. It makes me confuse and a bit worry....