Vermouth is wine that has been fortified with various roots and botanicals. If you're planning on making a lot of classic cocktails like Martinis, Manhattans and Negronis, you'll want to keep a bottle of each.
Note that vermouth is not shelf-stable so you'll need to refrigerate it after opening. Sweet vermouth will keep refrigerated for about 90 days, but the flavor will gradually change over that time. Dry vermouth tends to go bad a little faster but will keep in the fridge for around 60 days.