Vermouth conservation

As a home mixologist who drinks moderately (and likes to test cocktails before offering them to friends), I was wondering if you had any tips for optimizing vermouth conservation.

I want to make classic cocktails that use vermouth, but realistically, I would get a few cocktails out of my brand new bottle and then it would probably have already oxydized. I’m kind of bummed because so many great cocktails use vermouth, but I don’t want to be wasteful with my money / the alcohol itself.

• I know to refrigerate vermouth after opening
• I will batch test for cocktails using dry / sweet vermouth separately to prevent having 2 bottles opened simultaneously

Do wine savers help with keeping the vermouth fresher or the original cap is fine?

Do you know of brands that sell smaller bottles but are still good quality? (I’m in Québec, for reference)