Why is Vermouth so misunderstood? Is it one of the reasons cocktails, especially Martinis, have such a bad reputation?
Vermouth is a fortified WINE, like any other wine it goes BAD after a while.
It needs to be refrigerated after opening and used within 1 month of opening. In bars, it is usually at best at room temperature, at worst near some hot lights or hot water under the bar or in the well. At home, people seem to like to store it over the stove. Would someone do the same with wine? Or cream for coffee?
No, of course not. But they do it with vermouth. Might as well use vinegar instead.
Never buy anything bigger than a 375 ml (1/2 bottle) unless you are having a large party, refrigerate it after opening, and use it or dump it after 1 month.
Also, use a decent vermouth like Noilly Prat. Maybe it costs another 10 cents per drink but there is an immense difference in taste.