Can I change my F visa to X Visa in Shanghai,go to Vietnam and back into China on a student visa?

Hello.

I am currently in China on a F business visa.

I have been accepted to study at Shanghai University in February 2013.

I would like to leave the country (visit Vietnam, 2nd-15th February 2013) and enter the country as a student from Vietnam to start my study course.

Thank you so much!