  Substantial silicate vapor is expected to be in chemical equilibrium at
temperature conditions typical of the silicate-atmosphere interface of
sub-Neptune planets, which can exceed 5000 K. Previous models of the
atmospheric structure and evolution of these exoplanets, which have been used
to constrain their atmospheric mass fractions, have neglected this
compositional coupling. In this work, we show that silicate vapor in a
hydrogen-dominated atmosphere acts as a condensable species, decreasing in
abundance with altitude. The resultant mean molecular weight gradient inhibits
convection at temperatures above $\sim 4000$ K, inducing a near-surface
radiative layer. This radiative layer decreases the planet's total radius
compared to a planet with the same base temperature and a convective, pure H/He
atmosphere. Therefore, we expect silicate vapor to have major effects on the
inferred envelope mass fraction and thermal evolution of sub-Neptune planets.
We demonstrate that differences in radii, and hence in inferred atmospheric
masses, are largest for planets which have larger masses, equilibrium
temperatures, and atmospheric mass fractions. The effects are largest for
younger planets, but differences can persist on gigayear time-scales for some
sub-Neptunes. For a $10 M_\oplus$ planet with $T_\mathrm{eq}=1000$ K and an age
of $\sim 300$ Myr, an observed radius consistent with an atmospheric mass
fraction of 10% when accounting for silicate vapor would be misinterpreted as
indicating an atmospheric mass fraction of 2% if a H/He-only atmosphere were
assumed. The presence of silicate vapor in the atmosphere is also expected to
have important implications for the accretion and loss of primordial hydrogen
atmospheres.
