V-Ray material files are textures used with software like Sketchup, Rhino, 3DSMax, etc. Originally developed by ASGvis, the company is now owned by Chaos Group.
Original .VISMAT format is a binary file, later versions used an XML tagging structure which is interchangeable with the .VRMAT format. Tagging begins and ends with <vismat>.
V-Ray 1.49 was last version to write to the .VISMAT format.