It seems many scenarios are becoming possible where once traditional "recording techniques and setups" are being used for live audio.
 
(Soft synths , routing from digital mixer to VST as an effect send etc., live autotune VST etc.  )
 
The latency gap has been minimized to where many of these things can be done with a PC using software.
 
I have a keyboard in local control off mode and it connects USB to a DAW host (sonar) and it will route the DAW output back thru the synth for easy output to PA without extra audio cables (or even having an audio interface on the DAW. (other then the ASIO interface that is created by the yamaha MX-61).
 

 
So as the performer performs they have the keyboard hooked up to a laptop DAW, they arrow down to activate the track that has the VSTI they want to play, including a track that sends midi back to the local synth when they want to use those sounds.
 
Conveniently it all comes out the yamaha MX main outs into the PA.
 
How could you design a track that effectively layered a soft synth and the actual keyboard synth? 
 
In other words ...
 
can one create a track in sonar with Rapture/Dimension synth on it and have it "forward" or "thru" the midi to another midi channel ( the one playing notes on the hardware synth...the yamaha MX-49?)
 
this seems related to midi echo but no sure?

Just create a second MIDI track that echos the same input to the hardware port.

What Brundlefly said plus a reminder to keep a close eye on the MIDI input monitoring status of the tracks involved to make sure teacks only echo MIDI to hardware or software when you actually want them to.

It's possible to automate the process by e.g. a volume envelope on the MIDI track that sets volume (usually but not always CC7) to zero when you don't want the connected synth to play. Though this does lock you into the arrangement created in advance which may not always be desirable.

If you only need a single software synth (or none) and the bulk of processing is done in hardware the most cut-down version of Live can be useful for this. It's been given away with lots of interfaces over the years so you may already have a copy somewhere.

Update .  works great and gonna perform with it.   The yamaha MX has a steinberg driver that makes the synth show up as an audio device.  point the output of the soft synths to that.
 
I created a sonar project template with all the extra menus closed that just has a big view of tracks.  Then I created one midi track called yamaha that just sends the midi back to the synth, then all the other soft synth tracks.
 
Change tracks by arrowing up or down to change the "patch".  Then turn local control off on the synth so it does't play along with the soft synths. 
 
Voila!