03 Chevy Cavalier only getting 17mpg. Had line connecting to air intake dumping gas.?

I'm (yet again) working on my mom's 03 Chevy Cavalier and her gas mileage is terrible. It's only getting 17mpg and 2 years ago it was around 34mpg. I fixed a few issues for her and one of them was a split vacuum line connecting from the air intake manifold to some sort of gas pump/line. The split hose was spraying gas both inside the air intake and into the engine compartment. I re-ended the vacuum line and no more gas is being sprayed, and I hoped this would re-set the pressure solving the mileage problem but the mileage hasn't changed. I replaced the gas cap just in case.

Aside from the mileage sucking the car runs perfectly, absolutely no issues.

Does anyone know why the mileage could have been cut in (exactly) half? Does anyone know what the vacuum line does that was spewing gas? I'd like to know if it's now just dumping into the intake instead of spraying, but that's not easy for me to test especially without knowing how it works.