How do I stop vapor lock?

What he just said ^^^

Take a look at the original diagrams.

What I would do if I were you is the following:
Vent from the filler through the filler tube grommet under the trunk (this way it doesn't vent into trunk).
Then run your return (either from a vented filter or a regulator) back to the vent that's currently capped at the sending unit.

Connecting the vent on the sending unit to the vent on the neck won't do anything ;)