While vermouth is not bad from a health standpoint for a long time, it does lose much of its distinct flavor about six months after opening a bottle. Vermouth is meant to be consumed while still young, and a bottle should be consumed one year from its original production date.
To extend the shelf life of a bottle of vermouth, store it in the refrigerator once opened with its original cap tightly screwed in place. Smell the vermouth before using it to determine if it still has its original aroma, and if not, discard it. In the unlikely case that a bottle of vermouth has any off odors, discard it, as this is a sign of spoilage.