I need a substitute for Marsala cooking wine.

Agree with Cannizzo - the Vermouth will do fine.

Can I give a suggestion I used to make chicken Marsala my husband favorite until I discovered pork tenderloin if you cut about an inch thick and pound it down thin follow your Marsala recipe it looks just like veal I think even better please give it a try you won't be sorry

If you are making veal or chicken Marsala you are using dry Marsala I would substitute dry vermouth I have done it in a pinch and it worked fine

It really depends upon what you need it for. Sometimes sweet sherry or a port will do, other times maybe some kind of liqueur. Is it for a sweet dish or a savoury one?