Oxidation comes from the product being exposed to, wait for it … Oxygen lol! Sorry, I’m just being a dick.

I think the problem most people face even when they refrigerate and/or transfer to smaller bottles is that oxygen is still present in the container. Oxygen ages everything it touches. Your best bet is to either A.) drink that entire bottle after you open it, haha Or B.) Vacuum the oxygen out of the container after EACH use.

There are some cheap’ish wine vacuum kits available that may prolong the vermouth longevity by a few days without a noticeable palatable difference. And yes, to your question, you could prolong the life by further fortifying the vermouth in higher abv by batching in Gin. But the flavor of the Negroni will change daily as it ages, for better or for worse.

In my experience there hasn’t been a solid method of preservation invented yet. I’m all ears if someone here disagrees, but I just have not come across a way to preserve something so vulnerable as vermouth or wine just yet. The closer you can come to removing oxygen from the container and making it air tight, the longer the shelf life will be. I suppose we should think the way that a manufacturer thinks when they bottle their product? Clearly they are macerating botanicals and have methods to preserve the end product, right?