I believe it’s for fuel expansion not just vapor. The fuel in the tank can expand and then it goes into that overflow. Not certain, but maybe others can weigh in. I do know there’s nothing inside, there are two connections to the fuel tank, and one connection to that funnel shaped thing at the back of the car and that’s it. Those connections to the fuel tank really need to be tight and with good hoes otherwise you’ll get a gas smell in your trunk.