Vexillum cosmani is a species of small sea snail, marine gastropod mollusk in the family Costellariidae, the ribbed miters. It is a non-broadcast spawner, meaning the species reproduces through internal fertilization and carries the eggs until they are more developed before releasing them. Its life cycle does not include a trocophore stage.
The length of the shell attains 7.9 mm.
This marine species can be found in the eastern central Pacific Ocean and French Polynesia. It is mostly found off the coast of Hawaii.