VISA extention

Hi. there...
I'm just wondering about VISA extention.
Actually, I'm staying in Hanoi but my VISA will be expired on Feb.
So I'm going to apply the invitation letter(For VISA on arrival and have to go back to South Korea and then coming back here to Hanoi on weekend.
My Question is ...
Do I need to stay even on one weekday(working day) outside of Vietnam?
I'd like to go out from Vietnam on Saturday and come back here,Hanoi, on Sunday.

Thanks...