Dry vermouth is similar enough in flavor to white wine that we've found it to be a good and more shelf-stable substitute. We know that it can be stored for months at room temperature and still make an acceptable pan sauce or risotto. But we wondered how long an open bottle would retain enough aroma and flavor for sipping and cocktails and whether chilling prolonged its quality.
We stored open bottles of two different vermouths at room temperature and in the fridge. Tasters sampled them every other week. After a month, most tasters found that both vermouths stored in the pantry had lost their citrusy aroma and tasted flat. The vermouths stored in the refrigerator remained suitable for sipping for two months before they started tasting medicinal and somewhat bitter. Refrigeration does a better job of preserving flavor because low temperatures slow down the loss of aromatic compounds and the lack of light slows oxidation.
THE UPSHOT: If you're cooking with vermouth, it's fine to store it at room temperature for several months. For the best flavor in cocktails, keep the bottle in the refrigerator for no longer than two months.