Update:
The newest version of the ReaLearn VST can send and receive OSC (and Midi) messages, can convert them, can controöl Rteaper features by them and can send them out according to certain events in Reaper.
By this you might be able to avoid using an external executable altogether.
I don't know if that version already is available via ReaPack.
Please check out this forum thread: ->
https://forum.cockos.com/showthread.php?t=178015
-Michael