as andrew noted, you could write a weewx service to do this.
or you could create a serial port sniffer in hardware. create a wiring harness with 3 serial ports - one port plugs into the vp2, the other two do a 'Y' from that. one of those two plugs into the computer running weewx, the other plugs into the other listener. you only have to connect 3 wires: TX, RX, and GND. you can even eliminate the TX pin for the non-weewx device.
as long as weewx is the only software sending commands, the other device will be a passive listener.
this is a common approach for reverse engineering serial protocols, and it is an easy (and super inexpensive) way to get serial data to multiple devices.
but only ONE device can do the sending/commands. others must be passive listeners.
then write your software to read and decode the traffic that passes over the serial line.
m