The points have to be valid for the dates of your vacation. In other words, they have to be in the same use year as the desired vacation. So you would have to bank the points before you make the reservation.
Assuming you are within your banking deadline, MS can bank the points before the reservation is made.
I assumed when you said "this year's" and "next year" that you were referring to your use year. If you really meant calendar year, then we would need to know your use year and the month of your planned vacation to give you the absolute correct answer.