Vapor Lock question

Yeah, gotta agree, do not vent the cap.

Check the venting, if you're making some HP you may need to increase the size of the vent. Where exactly to put it depends on too many variables, but if racing is in the mix then have a good look at their rules book. A series that I crew in requires that there always be a part of the vent line that is above the fuel level no matter how the vehicle is rolled.

I like to put one of the plastic bodied cheap fuel filters in the end of the vent line hose. Keeps any road grit from getting in and either plugging the vent or getting into the fuel itself.