I find light wisps of vapor to be normal and will continue for a little while after shutdown.
I have seen moisture in the oil increase the amount for a while as would some low boiling contamination like gasoline,diesel,solvents, etc. but would lessen after running a while at good temperatures as these boil away.
Where you get concerned is when the amount increases greatly as the engine load/cylinder pressure/throttle opening increases.
I would expect my good engine, breathing lightly at idle, when the throttle is suddenly opened a increase in vapor would be not much, but noticible.It should be smoothly issuing not puffing which would indicate one cylinder leaking more at the rings than it's neighbors.If it's an automatic you can hold the brake while in gear and really load it for an observed test.
When a lot of blow-by is present, it can fog smoke out the draft tube and vented cap and become noticible behind the car and even in the cab---this will lessen with light load and increase with throttle opening/heaver load.RPM doesn't make as much difference as the load which is greater momentarily when throttle is opened.
The draft tube system is designed(?) to create a draft by air flowing over the bevel cut tube--so the valve cover slots are to let air in--when the draft is established.If no air across the tube--no draft so the engine breathes whatever it needs to at the vent providing the least restriction.The fan can provide an air flow over the draft tube, but not at idle.
Many older industrial engines used draft tubes and under load the issuance was a pretty good indicator of cylinder sealing efficiency.
