“Under certain circumstances”. Snow covering mushroom cap, mushroom cap pushed on too far, etc.
but then i guess the engine wouldnt start anyways.
Even if there were situations where a vacuum on the vent lines happened, would it do any harm? I guess if it was strong enough, you could pull some air or water through seals (theoretically), but that would need to be some serious suckage.
Do any of the vent lines enter below fluid levels? If so, maybe you could pull some fluid up a little, but again you'd need to have some air coming in from somewhere or the hubs/diffs would just be under continuous equalized vacuum (not sucking anything up).
I think it would be like putting a vacuum pump on the top of a bottle. As long as the bottle is sealed and rigid, the vacuum can't pull the liquid out of the bottle if the vacuum point is higher than the fluids (unless you get the vacuum so extreme if boils the liquids).
All that said, I know very little about HMMVW's and don't know how these particular vent systems work, so I'm just talking in concepts and what I know about venting on regular differentials.