Mix vinegar and 2 tablespoons chopped mint with 2 tablespoons water in a small nonreactive saucepan. Bring to a simmer, then remove from heat. Let steep 30 minutes, then strain into a medium bowl and discard mint. Add shallots and whisk in the oil in a thin, steady stream. Season with salt and pepper.
Mix romaine leaves in a large bowl with remaining mint, sliced red onions, and grapes. Add enough vinaigrette to coat and toss well (reserve remaining vinaigrette for later use). Taste, then season with salt and pepper if necessary. Arrange on chilled plates, top with the toasted almonds and slices of ricotta salata, and serve.