TCWV (Total Column Water Vapour) and VIWV (Vertical Integral of Water Vapour) are essentially the same, but produced by different software.
TCWV is produced by the IFS (ECMWF forecast system) and is available in the analyses and forecasts.
VIWV is produced by the ERA post processing budget software and, for ERA-Interim, is only available in the analyses.
More information is available in the ERA-Interim archive document:
http://www.ecmwf.int/en/elibrary/8174-era-interim-archive-version-20
Both TCWV and VIWV are in kg m^-2.
