VMAP
VMAP stands for Valve Map, and it is the main map file extension used by 
Source 2 Hammer. The file itself is just a DMX file under binary encoding version 4, however Hammer can export an ASCII-encoded vmap by clicking "Save Copy As Text".
Some key differences with preceding formats:
 Source |
Source 2
| |
|---|---|---|
| Geometry uncompiled | Brushes in Valve Map Format | Meshes in half-edge format |
| Geometry partitioning | Node and leaf lump / BSP | .vwnod_c file / octree |
| Geometry compiled | Face lump | .vmdl_c files |
| Visibility compiled | Visibility lump | .vvis_c file |
| Physics compiled | Physics lump | .vphys_c file |
| Entities compiled | Entity lump | .vents_c file |
Compiled VMAP
VMAP_C stands for Valve Map Compiled, and is the main map file extension used to load maps in Source 2. The .vmap_c file is used to refer to various map dependencies (including vwrld, vwnod, vvis, vphys, vents, vtex, vmdl, vrman; usually residing in maps/<mapname>/) as well as other external assets used in the map.
This file along with its main dependencies usually get bundled together into a single VPK file, the latter of which serves similarly to the .bsp. Since lumps are separate files, they get packed and live next to other assets in the VPK. This makes post-compile addons much more convenient than in Source (see Patching EP1 levels vs. HL:A Map Extensions).