How can I get a re-entry visa in China?

I am a student in Beijing. I am planning to visit Vietnam for 7 days in early October. I obtained my student visa in Hong Kong but they were not able to provide me with a double entry visa. Now that I am Beijing, how can I get a visa which will allow me to leave Beijing, enter Vietnam, return to Beijing then return to the US. I currently have a student visa into China (obtained in Hong Kong) and leaving in mid-December.