Not sure I would worry about fuel coming out the vent line.
Here's a section of a tall pump I saved that was published by High Performance Pontiac.
Obviously there's no guarantee your pump is built the same but probably similar, even if a short can.
I am no expert but looks like the vent line was connected to an internal tube that had a small orifice sitting in the top of the pressurized outlet chamber.
If I'm wrong, I apologize but think it would be normal for gas to flow in the vent tube, just not high volume because of the small orifice. If vapor built up in that big chamber, it could vent through the line back to the tank, which I think was the purpose.
If you think the gas in the vent line is an issue, maybe cut the pump open and see if you can figure it out? Maybe that tube broke off?
If the pump was otherwise working normally, the only other thing I could imagine is if gas somehow backed up from the tank into that vent line and headed toward the pump. I don't remember where it exits on the float assembly but is there a chance your tank isn't venting and pressure is building up in the tank?
