What to substitute for sherry?

Actually, I would go with white (dry) vermouth as the red vermouth will be too sweet . But, if you want sweet, use red vermouth, rube sherry, or port.
Using white vermouth will allow you to adjust the flavor and sweetness. Once it is too sweet you have a bigger problem.
Although a bit rare, there is white port which is more dry than sweet. Nice with cheese for dessert too.

Make sure it is sweet vermouth! The stuff for Manhattans, not dry vermouth, the stuff for Martinis. For quiche mushrooms, you could deglaze with any wine, brandy or even stock, though the flavor profile will be different.

Oh cool! I have vermouth. I'm Sauteeing some mushrooms for a quiche. The recipe called for sherry. Thanks!

If for cooking, marsala or port could work. Even sweet vermouth depending on the dish.

For drinking or for cooking? If for cooking, what are you cooking?