Don’t Use Old Vermouth
There used to be a time when the amount of dry vermouth that would make it into your Martini would have been better measured by an eye dropper instead of a jigger. To this day, you can still find little spray bottles being sold as “vermouth misters” to allow only the slightest amount of vermouth to be added to your Martini. When you are using that little vermouth in your Martini, that means that you are going through your vermouth very slowly, making it very, very old before you make even the slightest dent in it.
Vermouth is a wine. And like any wine, it will oxidize over time, which will impact its flavor. Vermouth is what is known as a fortified/aromatized wine (Port and Sherry are simply fortified wines). Fortification simply means adding an alcohol to the wine, usually brandy. This originally was done to help preserve it, the higher alcohol content would make it last longer. Aromatization means that herbs, spices, and botanicals have been added to it. The original intent of this was to produce a supposedly medicinal beverage, with wormwood being the key ingredient of vermouth, which is where it gets its name. These botanicals also had a side-effect of giving the wine a longer shelf-life, not because it reduced oxidation, but because it would sort of mask the effects of oxidation. Even with fortification and aromatization vermouth is still a wine, and so its shelf life, once opened, is limited.
Those dusty bottles of vermouth you might have on your shelf are not going to do anything good for any drink you use them in. This could be part of what leads to the fear that some people have of vermouth, and hence the gymnastics they may go through to use as little of it as possible in their cocktails (the Martini specifically). You owe it to yourself, and the guests you are serving, to use as fresh of a bottle of vermouth as you can. This will mean buying as small a bottle as possible and keeping it refrigerated when not in use. If you have any doubts about the age of that bottle, then relegate it for use in cooking, where it works quite well.
1 1/2 oz gin
1/2 oz dry vermouth
dash orange bitters
Stir ingredients with ice.
Strain into a chilled cocktail coup.
Garnish with a lemon twist.