And as a result, your Negronis and Manhattans taste funny.
I'm often asked for home bartending tips, and there are dozens. "Ice matters a lot, "You shake too much and stir not enough," and "Always use fresh citrus" are up there. But the number-one flaw I see, time in and time out, is buying a bottle of vermouth (be it red or white), cracking it open to make a drink, putting it back in the cabinet, and expecting that it will stay usable as if it were a bottle of vodka. Unlike vodka (or gin or scotch), vermouth is not a spirit but a fortified wine, which means it has a whole lot more in common with that bottle of Poplar Grove you just opened than it does with the bottle of Laphroaig your uncle gave you last Christmas. The rub is, there's no real solution. Putting the opened bottle in the fridge helps, buying smaller bottles helps (for some reason, vermouth is sold mostly in one-litre containers when it should be sold only in 375 mls), or transferring the bottle into smaller containers with no air all help. But the truth is, you have to drink it. When I'm in the States, I stock up on minis (airplane size) of vermouth whenever I see them. And if I'm buying a pricey bottle like Carpano Antica, I'll split it with friends, each of us taking a third. Even with the above, you'll only get maybe 45 days out of a bottle; after that, your martini is going to taste a little off. It won't hurt you, but it sure won't taste like the one the pros make either.