Youd be hard put to notice any extra vibration. There's already enough to go turn your coffee frothy.
The "dent" is probably a stone chip from landing (reverse thrust drags a lot of dust and debries up) the engineers will then have blended out the chip to prevent cracking.
Its very common and there is no overall effect on the aircraft performance.
We had loads of them and only 2 of our aircraft did low level so its not just errosion from salt water.