Comment by ButlerianJihad

I own a vacuum-insulated flask that is not Thermos®. I purchased it with a one cap and immediately purchased an accessory cap.

The original cap from the store has no pressure-release mechanism, other than the breaking of the seal when unscrewing it. The cap has an integrated carrying handle, which can also help when turning the cap one way or another.

The accessory cap has a "sippy cup" feature along with a pressure-release valve. It is very common to have a second valve with any vessel that pours or dispenses liquid, even a lemonade tank, because that's how pouring works. When you remove liquid from the container, it must be replaced by a commensurate volume of air.