This sounds like the effect from a diffraction grating (a finely ruled shiny surface). The diffraction gratings used in spectroscopy have parallel grooves etched in the surface that reflect and disperse light into its constituent wavelengths.
It does, but I'm assuming a vent doesn't have the kinds of gaps that are that narrow.
Refraction is when waves, whether physical or electromagnetic, are deflected when the waves go through a substance.
I assume that something is being vented - possibly water vapour/steam - thus the possibility of a 'rainbow' effect.