Travel visa for Vietnamese living in France

Hi. I've gone to China several times. This time, as a student in France, I want to apply for a Visa to China for Christmas vacations. I have booked the ticket to China. However, I'm still indecisive on my return day. It is either the 1st January or 6th January. Thus, I did not book my return flight. Is that possible if I apply for a visa without a return ticket?

Besides, I will stay with my friend who is a student in Changsha. Could I ask him to issue an invitation letter instead of having to provide the proof of hotel booking?
Thanks