For a person who writes and edits a website about drinking, I must admit that I do a terrible job of finishing a full size bottle of sweet vermouth before it goes bad. Don't get me wrong—I love the stuff, especially smooth, rich Carpano Antica Formula. I love it in a Negroni, I love it in a Manhattan, and I really love it on the rocks (sometimes with a squeeze of citrus.) But a liter bottle of Carpano is a lot to get through before it starts oxidizing, even if you keep it in the fridge. I'm always heartbroken when I realize I've let it go to waste again.
Some people swear they can preserve vermouth by mixing it into cocktails in bottles. It probably helps that these are sealed tight, with little headspace, unlike your half-full bottle of Carpano, but the better answer is just to find a smaller bottle and drink it faster, before it goes bad. Buying a liter of vermouth is not that different from buying a gallon of milk: if you don't have a house full of thirsty drinkers, you're better off sizing down. And you don't save any money if you're pouring it down the drain.
So I'm on a campaign to convince more stores to stock the wee bottles. Currently, you can find the 375-mL bottle of Carpano (as well as Cocchi Vermouth di Torino, Byrrh, and the full Dolin range) at Astor Wine & Spirits, Ambassador Wine & Spirits, and Mister Wright Liquors in New York City. In San Francisco, you can find the small sizes at Cask, D&M, and K&L Wine Merchants.
Have you spotted small bottles of vermouth where you live? Do you use 'em, or do you have any strategies for getting through a full liter before vermouth goes bad?