bought a round way ticket from KOREA to VIETNAM transit at Beijing

Dear Sir/madam,
My name is Thu, i'm Vietnamese and i am working in Korea. I bought a round way ticket from KOREA to VIETNAM transit at Beijing. At return way to back Korea, i will transit at Beijing for 20 hours. I want to get Chinese transit visa to travel in Beijing on 20 hours before i back to Korea. I have Korea's alien registration card also. So, can i get transit entry permit at Beijing airport, or should i apply it at Chinese embassy in Korea? Thank you so much.