Normally a VN series is looking for signal from the vending machine (enable) which tells the bill acceptor the machine is ready.
Operation of the bill acceptor outside of a machine (like on a test bench) would require certain pins on the side of the bill acceptor to be jumpered thus enabling the bill acceptor to take money.
On the AE series, there is simply a dip switch to set it for being "always enabled" or have the host machine "harness enabled". On amusement and jukebox applications the AE series is usually set to always enabled with the dip switches.