Didn't we just have a conversation about people wanting to use a wine other than Marsala to make chicken Marsala? It's a unique wine, so I don't think there's a substitute that will have a similar flavor.
You could try this, but I have no idea how similar it is.
1/4 cup of white grape juice
1 tablespoon vanilla extract
2 tablespoons of sherry vinegar
Anyplace where people argue about food is a good place.
~ Anthony Bourdain, Parts Unknown, 2018