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).
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.
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.