It's a slit in a garment, usually in the lower edge of the back of a coat, to allow easier movement.
Or, it's an opening through which gases, especially air, can pass. Or the opening of a volcano from which lava flows.
Take your pick
(1) If the vent in your jacket is emitting lava, you've overheated.
(2) I let it out at the vent, or it let me out, depending on your point of view. Anyway, it fits better now and I don't feel bloated any more.