There is only one file type that uses the .VMAP_C extension:
Valve Source 2 Compiled Map File
A VMAP_C file is a compiled map file used by games with the Source 2 engine, such as DOTA 2 and Robot Repair. It contains a map, which includes the layout of lanes, ancients, fountains, barracks, runes, towers, neutral camps, shops, and other map elements. VMAP_C files are compiled from .VMAP files using Hammer Editor, which are then used in the game running the Source 2 engine.
VMAP_C files were introduced with the release of the Source 2 engine and replaced the .BSP file, which was used by Source 1 engine games. VMAP_C files are most commonly used by DOTA 2 but may also be used by other games that use the Source 2 engine.
You can create VMAP_C files with the Valve Hammer Editor, which is included in the Source SDK and DOTA 2 Workshop Tools. After you are done modifying a VMAP file, you must build it into a compiled map (VMAP_C file), which is packaged in a .VPK file. Select, choose your settings, and click to compile the VMAP file.
Updated: September 15, 2017