You need the air so that the tobacco will mature (age) for you. No air and you'll end up with pretty much what you started.
Air (oxygen) allows aerobic activity ... be it fermentation with yeast or bacteria ... the lack there of brings upon anaerobic activity provided the proper critters are present ... which is what happens after all or most of the O2 is depleted. All a vacuum does is reduce the amount of O2 for aerobic fermentation. And again, I have not read any "research" talking about which fermentation does what to any tobacco ... I wonder if the pipe tobacco industry would fund such a project? Probably not ... they already have us ageing tobacco for them (cellaring) so that they can sell it before it peaks saving warehouse space ...
