As a fortified wine, vermouth can spoil, but the taste dulls over time prior to it going bad. Vermouth should be consumed within six months of opening the bottle as it is meant to be used when fresh.
Proper storage of vermouth is necessary when preserving the quality. Vermouth is to be kept in a dry place, away from direct sunlight. Vermouth needs to be stored in the refrigerator if it is not going to be consumed over the next few weeks to help maintain the taste. If using the bottle over a shorter time frame, storing it in a cupboard or liquor cabinet works well.