They aren't, and the driver's side vent window happens to sit open enough (it disconnected itself from its arm) that the interior atmosphere and the exterior would still have a go-between if it was.

Your theory about stress from the rust bubble is a good one; might prove to be correct when the vinyl comes off.

Mind you, there is a semi-central point to this crack, but no chips indicating the slightest impact of anything.

-Kurt