A toilet will flush without a vent. A clogged vent will NOT siphon a toilet dry. Air movement in a vent can rock the water in the toilet, but would not siphon it dry. The rocking could cause some to slosh over the outlet weir, but it would take some really high winds to affect the level much. That hole may have been for hot gasses to escape during the firing in the kiln...it likely should have been plugged prior to leaving the factory, and it may be that it was not done properly and fell out sometime and you noticed the level drop.
Replace the toilet, or if you're feeling lucky, try plugging that hole. But, if you saw a hairline crack, stop wasting your time and replace the bowl, or better yet, the whole thing.