Line 2 baking sheets with waxed paper. Combine 1/2 cup cold water and the vanilla in a mixing bowl. Sprinkle gelatin over top, and let stand until softened, about 5 minutes.
Bring granulated sugar, corn syrup, and remaining 1/4 cup cold water to a boil in a small saucepan. Cook, washing down sides of saucepan with a wet pastry brush, until mixture registers 234 degrees to 240 degrees on a candy thermometer. Remove from heat.
Whisk gelatin mixture with a mixer on high speed for 30 seconds. With the machine running, carefully pour hot sugar mixture down the side of bowl in a slow, steady stream. Whisk mixture until very fluffy and almost stiff, 8 to 10 minutes. Transfer to a pastry bag fitted with a 1/2-inch round tip.
To form the bones: In a continuous motion, pipe a small figure 8, then a straight 5-inch line, then another small figure 8. Repeat. Let bones stand, uncovered, for 10 to 12 hours in a cool, dry place.
Generously sift confectioners' sugar over bones, turning to coat. Shake off excess. (The marshmallow bones will keep in an airtight container at room temperature for up to 2 weeks.)