yes, your drawing shows the right idea. after you empty the bag of air around the vacuum, close the valve and slowly open the thermos to inflate the bag.
the volume you can calculate yourself using the density of liquid nitrogen / molar mass, then the gas law.
you would be able to regulate a little with the valve, I would keep the thermos closed for as much as you can, but yeah, you could add some volume to the bag when needed.
sorry I cannot be online more after this message, but I have to run