Figure 1: Typical mid-latitude TOA emission of radiation from earth. (original graph is from NASA )
A common observation of thermalization by way of water vapor is cloudless nights cool faster and farther when absolute water vapor content of the atmosphere is lower. Clear nights cool faster and farther in the desert than where it is humid.
At high altitude (above about 20 km), energy is conducted from non-ghg molecules to ghg molecules for radiation towards space. For lack of a better term, call it reverse-thermalization.
Ghg in the warmed air can emit photons only at a limited number of wavelengths (or wavenumbers) characteristic for each molecule species. Furthermore, all theoretically possible wavenumbers are not equally likely.
Most of the photons emitted by the water vapor molecules are at wavelengths different from the comparatively narrow band that CO2 molecules can absorb. Effectively, much of the terrestrial thermal radiation energy absorbed by CO2 (and other non-condensing ghg) is thermalized, redirected to, and radiated to space from water vapor.
At very high altitudes, temperature, molecule spacing and time between collisions increases to where reverse-thermalization to CO2 (and O3) molecules becomes significant as does radiation from them to space.