Combine grapes, onion, jalapeño pepper, cilantro and vinegar in medium bowl. Season, if desired, with salt and pepper. Let stand at least 30 minutes.
Preheat oven to 375 degrees. Arrange bread on large baking sheet in a single layer and bake, turning once, 8 minutes or until lightly golden brown and crisp on both sides. Remove and let cool.
Combine goat cheese with mayonnaise in small bowl until smooth. Season, if desired, with salt and pepper. Evenly spread cheese mixture onto crostinis, then top with grape salsa.