It is possible to send a message from a specific keypad on Ademco/Vista or to a specific partition on DSC.
Its an Ademco Vista 20P.It does not have a serial interface, it just has the keybus interface.
Keybus is a proprietary protocol thats a modified RS485 protocol (its basically RS485 with clock signals). My solution was to drop an optocoupler into the circuit before adding my serial interface. Its slow data, 4800 bps (N81). The data is reversed, so be aware of that.