It should be no problem if the vial is closed very well. It is very important that you wait at least 30 min at room temperature, before you open. Otherwise, the sample get moisturized very quick. I would not drop the vial right into the nitrogen tank, because the plastic gets very brittle at that temperatures, may break. Use a sample holder to keep it not floating around. You could also place the vial in the gas space above the nitrogen, this should be sufficient enough to keep it cooled well.

Alternative would be storage in dry ice, which is "only" -80 °C.