Mine does the same thing sprayed carb cleaner in it but didnt help.
I think the brotheres on the forum sain put some seafoam on it, I think im gonna try that.
It dosent hurt mine because it dont make any difference in the running. Maybe im wrong.
Some vacuum in the tank is normal. As long as it doesn't affect engine performance, you will be fine. If performance starts to suffer, and you suspect the cap may be bad or plugged, try running it without tightening the cap all the way, to see if it improves.