you can have a try to apply for a Z visa in Ho Chi Monh city. But there is risk that you may not be granted a visa there, because Chinese embassy in most countries do not accept visa application from temporary visitors as that in Hanoi. Or, you can try to apply in HK.

Thank you. I understand it might be possible to get my Z visa in HCMC; however; if it can't be granted, I would also like to know if I can still use my current visa which is valid until mid-February to re-enter China before it expires?