I certainly wouldn't expect to be able to detect any pressure of vacuum signal from the puke tank. Or even measure such a change with sensitive equipement. On a normal system I don't see any reason why the trans or the rear end would "puke" or "draw" from such a vent.
It would have to be a rather catastrophic event for either the trans or the rear end to "puke", that vent is pretty high up in the case for either item.
As it concerns the case(s) I'm aware where the rear end DID puke into the trans it was determined that the rear cover was the fault. The wrong design, the wrong positioning of the vent, which was tee'd to the trans. An over filled rear end. Even then, one could speculate that was not the actual cause of the rear end running out of
oil and destroying itself. But it does beg the question: WHERE did that
oil go and WHY did enough of it leave the rear end to cause a gear failure and end up in the trans??
What would have to happen for a trans or a rear end to start pushing it's
oil out the vent?