Global Warming, page-182

There are no perfect vacuums in our atmosphere, at least with the exception of the space between the ears of an common garden climate denier.

In the real world N2 and O2 are not excited by infra-red radiation due to the arrangement of the atoms in the molecule, for this reason they do not contribute to what we call the greenhouse effect.

Water vapour is a potent greenhouse gas but it only stays in the atmosphere for a few weeks before returning to the Earth as precipitation. The same can't be said for CO2 which adds to the greenhouse effect for a much longer period of time and relies on carbon sinks (forests and oceans) to eliminate concentrations in the atmosphere.