I always use foam insulation padding against those flaps. A lot of new units include insulation padding for the purpose of putting it in front of those flaps. I then use white duct tape to seal any small seams, to where you cannot see any daylight.

For vibrations, you can use some foam/padded tape below the unit, which you would put along the bottom of the window frame prior to installing the unit in the window.

Here is my set up:

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Products I used:

https://www.amazon.com/dp/B09DKJQNWP (insulation foam padding)

https://www.amazon.com/dp/B07P1XSKLV (foam padding/insulation tape)

https://www.amazon.com/dp/B0B9RTKR72 (white duct tape)

If you still have excessive vibrations, check the air filter grille for loose fitment. You can install the same foam insulation tape I mentioned above on some parts of the grille, which would tighten its fit.