I doubt that is possible without modifying the game source.
Usually modifying the vanilla does not allow to host the resulting modification as any of the vanilla game types, you would need to at least add a + to the game type, so DM+, TDM+, CTF+, etc.
Without modifying the game source, it is possible with an econ bot that connects to the vanilla server using the telnet protocol.
The econ bot could react to the commands that are executed by the specific vote. You may put invalid commands that can be voted.
Those commands are executed by the server once they re voted by the players.
The server will try to execute them, but will fail, leaving a logging line in the external console that can be parsed and evaluated by your econ bot.
The bot needs to somehow now what maps are available on your server and then randomly choose one of those maps randomly and construct a new command "change_map xyz.map" that can be then executed by the bot via the econ connection, forcing the server to change the map to a random one.