$parallaxmap

The $parallaxmap VMT parameter specifies the VTF texture to use for the 'sub-vertex modelling' or parallaxmap of the material surface.

Warning:parallax maps are not currently supported in the Source Engine. I'm only guessing this is how it would work.

Basic VMT Syntax

"$parallaxmap" "Path/ParallaxmapA" 

• where Path is the filepath relative to game_directory/materials folder.
• where ParallaxmapA is the filename; ie ParallaxmapA.vtf.
• Notice the .vtf file extension is not used in VMT texture references.