Challenge

The description made a point of saying the author keeps Vim open all the time. That is the whole hint. An open Vim session leaves a swap file on disk.

Solution

Vim stores unsaved buffer state in a hidden .swp file next to the file being edited. The server ran server.py, so the swap file is .server.py.swp. Pull it and read the raw bytes:

hexdump -C .server.py.swp

The flag is sitting in the dump as plaintext.

Flag

bcactf{h!dDen_f!le$$!!_3bh5j7}