Part of the issue, Johnnyboydeer, is the breakdown of the oils. They break down in heat, and turn to glop. Different vegetable oils have different temps. That's why some oils are good for deep frying, and others not.
Have you ever felt the sticky on a cookie tray after using PAM on it? Hard to rub that off.
For lubing your cooker, an oil that doesn't turn to glop is best. Vegetable oil can't stand up to the temps that a cooker generates, so the oil doesn't get sticky and freeze your damper or something.
For your cast iron, vegetable oil works fine. If you were to put your frypan in a bonfire, you'd probably burn off all the seasoning. The temps are too high. But part of what is happening under normal circumstances is the oil is making a coating on the iron to protect it. It's no longer slippery grease (or else you could just wipe it off). The oil actually bonds to the metal surface.
Even so, it's recommended that for seasoning you use oils that are high in saturated fats, and therefore less likely to become rancid.