Vertex

Vertices are nothing more than coordinates on the map. Linedefs and segs reference vertices for their start-point and end-point.

The vertices for a map are stored in the VERTEXES lump, which consists of a raw sequence of x, y coordinates as pairs of 16-bit signed integers. The bytes are in little-endian order (least significant byte first).

Due to the 16-bit signed two's complement format, each coordinate can range from -32768 to +32767.

Lump size
The number of vertices displayed in an editing utility is less than the size of the VERTEXES lump divided by 32 bits. This is because the map designer cannot manipulate the extra vertices required for all segs to have endpoints; these are generated deterministically by the node builder (and then concealed from the user by nearly all mapping programs).

Doom 64 EX format
Doom 64 EX stores even 32 bit = 4 byte integers per direction, so each vertex uses 8 bytes. Doom Builder 64 reads them as long integer and divides them by 65536; internally it uses float values.