Vacuum baggers can seal the bag before the vacuum is fully formed. Just hit the button, and the heating elements will come on and melt the plastic. This is what is done for things like stews, where the liquid would otherwise be sucked out the bag by the vacuum pump.

Myself, I've used Tri-X 4x5 that was frozen in 1987 or so. Still good after opening the factory sealed bag. The sheets that were in a bag that had been opened were still usable, but 1/2-inch on the edge toward the bag opening didn't develop well.