Yes, you need to remove the vent tube to find the plastic tube that's inside the tank. The vent tube goes on the outside of the tank nozzle, and the plastic tube is on the inside of the same nozzle.
BTW, don't mean to discourage you, but I did the whole thing, finally managed to pull out that inner plastic tube with long-nose pliars, bit by bit, slowly slowly, when it just popped out after two inches! It was only two inches long!!!
Now, the cut was factory-made, perfect, so I doubt the previous owners did the modification - I think that my truck, a 93 Renegade for export, was just given a 15 gallon tank. The back lights have 1989 stamped into them.
Or maybe someone has another take on this?