I investigated this issue with a different VST3 supporting DAW and the symptoms I had seen with the S70XS mirrored more like that seen by user grnbriar in this thread http://forum.cakewalk.com/Sonar-X3-and-Motif-XF-VST3-m2910281.aspx 
 
In Sonar preferences, the additional synth USB midi ports, particularly port 4 must NOT be checked as midi IN or midi OUT ports. With these ports checked, there is no communication between the VST Editor and the synth, with them unchecked, then the VST Editor can control the synth.
 
So, currently the situation for the Motif and the S70XS is that the VST Editor can control the synth and its parameters, however keyboard data cannot be passed through the Editor.
 
The workaround for this seems to be to use the synth USB port 1 midi port rather than the VST ports for both midi in TO Sonar (if using as a controller), and midi out FROM Sonar to the synth. ie "synth -1" rather than "synth VST -1".  This also works also for recording arp out from the synth.
 
So, in summary, try this:
1. Ensure the only USB midi port checked in Sonar Preferences Midi (in & out) is port 1
2. instantiate the VST Editor
3. In the VST Editor midi source track, change the midi output assignment to the synth usb port 1 (in my cases its S70XS-1)
4. Add audio track, place it in the Editor folder and assign its input to your synth audio return.
 
Example below:
trk 6 : midi source on ch 1 to synth directly
trk 7 : arp output on synth ch 3
trk 8 : audio return on (in my case) motu 828 mkIII hybrid ch5/6
 

 
 
hope this helps someone get going...

I stumbled on this thread and I have updated the post at motifator with some more info.  I have the same problems with both Studio One 2 and Sonar X3 with the VST Editor.
http://www.motifator.com/index.php/forum/viewthread/469746/P0/
 
Hopefully Yamaha fixes this issue.  I see that Phil Clandestine took offense when you suggested that its functionality is restricted in DAWs other than Cubase, however I agree with your thought that Yamaha seems to have done this.

I looked through a bit of that thread and it seems that they are implying that  there is some kind of API for "external VST3's". In all my reading of the VST3 spec I did not come across anything like that. AFIK there is no published way for a host to know that a physical MIDI port is in use by a VST3 instrument.
 
I suggest having a developer contact from Yamaha that makes the instrument contact us, so we can understand better what they are expecting from the host.  If they need SONAR X3 for testing please have a representative contact us via this form and we'll be happy to send them a NFR copy.

Thanks for that,
With the write up I had also made a vid of the workaround here https://www.youtube.com/watch?v=cDCprqddPlg (which you have probably already seen) and it seems to have helped a number of folks. Since the end result is a solid workaround then the synth is working in every aspect that I need, so I stopped pursuing an elegant solution with Yamaha.
 

rlared
I stumbled on this thread and I have updated the post at motifator with some more info.  I have the same problems with both Studio One 2 and Sonar X3 with the VST Editor.
http://www.motifator.com/index.php/forum/viewthread/469746/P0/
 
Hopefully Yamaha fixes this issue.  I see that Phil Clandestine took offense when you suggested that its functionality is restricted in DAWs other than Cubase, however I agree with your thought that Yamaha seems to have done this.

Yeah the main thing is that as long as the Port 4 is communicating, it's pretty much equally as functional with the workaround.  To be honest, I bought the MOX thinking that I was going to use the on-board sounds for recording more than I do.  I find it easy to select a sound with the VST Editor, but for any sort of tweaking I don't like the interface and reach for one of the included instruments in Sonar instead.  But the MIDI/audio over USB is amazing at least.