Vapor Lock question

You're not getting vapor lock, you're getting more of a vacuum lock.
It's got to effect performance.

Does the inside of your cap look like the picture that 67Dart273 sent ?

Take your cap and line the inside with a paper tissue, then put your mouth on that tit and suck on it.
Do you get any air ?

My 2 cents.

That's a nice looking shifter. What Brand is it ?