How long does it last? Is 5 years (opened) ok? Should it have been refrigerated? I only need 2/3 cup for cooking with it & don't want to buy a new bottle or use a cup of white wine instead (that, I prefer to drink).
Vermouth will oxidize once opened. An open bottle will go bad after 2-3 months tops. An open bottle in the fridge will last probably 8-10 months. If you are just using to cook it it could be ok, but will probably have a slightly off flavor.
If is has been opened and not refrigerated, it is likely flavored vinegar. Depending on the recipe, you can probably find a cheap vermouth that will taste much better than the old stuff. White wine may work, again depending on the recipe.