Hydrogen bonding among water molecules is largely responsible for non-ideality in water vapor. Spectroscopic observations in pressurized water vapor using either IR or Raman techniques provide evidence of substantial molecular aggregation up to the critical point and above. The mean size or the molar fractions of the individual aggregates which are formed are, however, very difficult to determine from the analysis of spectra. Present paper aims at the spectral bandshape modeling in sub-critical water vapor making use of available IR absorption spectra in the OH fundamental and overtone. The modeling is supported by quantum-chemical ab initio and anharmonic vibrational calculations for the water dimer, the results of which are compared with previous ab initio and low-temperature laboratory data.