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.Vmap stands for Valve Map File, and it is the main map file extension used by Source 2's Hammer. The file itself is just a binary dmx file under encoding version 4, however Hammer can export an ASCII-encoded vmap.
The specifications for it are not currently known.
Vmap - Valve Map File (Successor of .map/.vmf) Vmap_c - Valve Map File, Compiled (Successor of BSP Format)