Cowley et al. (1969) listed it as an A-type subgiant star with a stellar classification of A7 IVn,[4] where the 'n' notation indicates "nebulous" lines due to rapid rotation.

The luminosity class of IV suggests the star has exhausted the supply of hydrogen at its core and is evolving away from the main sequence.

It is a Delta Scuti type variable star,[6] with a dominant pulsation period of 0.1881 days and an amplitude of 0.016 in magnitude.

[10] This rapid rotation is giving the star an oblate shape with an equatorial bulge that is an estimated 16% larger than the polar radius.

[15] Observations since 1997 suggest 21 Vul has an orbiting disk of gaseous material that is too equatorially confined to make it a shell star.