Sweet vermouth and dry vermouth have different sugar contents, so they don't age at the same rate, and generally, sweet vermouth lasts longer than dry vermouth. MasterClass says that since dry vermouth is wine and not a spirit, it will begin to deteriorate in flavor once opened, and should be used within a certain timeframe.

If your bottle of dry vermouth is only used for cooking, it will stay usable for a few months stored at room temperature before turning bitter. If your vermouth is solely for drinking, a bottle is really only good for drinking straight or in cocktails for one month when stored at room temperature, so the fridge might be a better way to go.

Cook's Illustrated compared bottles of vermouth stored in the refrigerator versus those stored at room temperature and found that vermouth lost flavor more quickly after a month out of the fridge. The bottles in the refrigerator, however, had twice the lifespan thanks to the lower temperature and the lack of light exposure.