It isn't just vaporized coolant. As the thermostat opens and closes, varying amounts of coolant are drawn into and out of the system. That's why there's such a large difference between min and max. As long as it's not overflowing or blowing out any of the tops, you're good to go. Just keep an eye on it. If it is ever really high, bleed some of it off and see if you ever drop below minimum. If you do, add more, if you don't, forget all about it.