Slice strawberries lengthwise about 1/8 inch thick. Lay the slices on paper towels and blot their surfaces gently without crushing the fruit.
Stir mascarpone with powdered sugar and vanilla just until smooth. Avoid prolonged beating, which can make the cheese grainy.
Spread the mascarpone mixture evenly over one face of every bread slice. Cover six slices with a single overlapping layer of strawberries, trimming pieces to fill gaps.
Close with the remaining bread, spread sides inward. Trim crusts with a sharp serrated knife and cut each sandwich diagonally in both directions.
Yield 24 small triangles, two per serving. Cover and refrigerate promptly, then serve within 4 hours so the berry layers and bread stay distinct.