i thought i once saw something about a simplesynth external ??
ANYWAY... here's how i got virtual synths running with pd:
in audio / midi setup (system preferences), activate the IAC driver
set pd's midi output to IAC , and the virtual synth's midi input to IAC
use your usual bag of [makenote] [noteout] [ctlout] etc objects to
send midi msgs.
this allows you to control standalone vsti's, or you can run vsti's through some other host software (like RAX)
however, the sound output goes directly to your soundcard or whatever.
if you want to get the sound from the vst into pd, you will need to use soemthing like JACK or SOUNDFLOWER (which i found to be quite good. you can get it from cycling 74's website)