Re: KM Americana.
Try wetting the gasket before fitting it. If that doesn't work, you can sand it down but it's not something I recommend doing unless you have to. I would personally rather find a better fitting gasket.
Most vases wobble. Try using a vase protector if you can find one that fits yours. If you got the Jumbo vase, a small vase protector would fit better than the large.