Building both pressure and vacuum is normal operation in a fuel system with a vented cap. The cap is not just "open" to the atmosphere all the time. It has a valve that must be overcome with either pressure or vacuum in order for either to be released.
If you don't have a vented cap, you should get one. Also, real vapor lock is pretty rare......although it does happen.
I recently had trouble with my truck and thought I was experiencing it, but it ended up being something else.
Any time you have fuel issues, you should first inspect the TANK, as it is the source. If it is dirty, it should be cleaned thoroughly, as any repairs without first cleaning the tank will be in vain.
