[Elecraft] AFSK A RTTY with VOX?

[Nick-WA5BDU]

Is it possible to have VOX control over transmission while in the AFSK A 
mode on RTTY and using the Line In jack for audio from the soundcard?  I 
tried unsuccessfully to do this in the recent RTTY Sprint using MMTTY.  
I finally had to use HRD with DM780, which I assume was controlling the 
T/R switching via the PTT.

Here's how I've set things and troubleshooting I've done -

Mode: Data; Data mode: AFSK A
Soundcard out connected to Line In on rear panel.
VOX is ON
TX Standby is not enabled
MENU: MIC SEL is selected to LINE IN
MIC Gain set to a nominal non-zero value
Verified that the sound card is putting out a strong AFSK signal.  Also, 
by pressing XMIT, the K3 does transmit this signal and give the required 
bars of ALC indication.

Should I be using the microphone input to have VOX control?  I wouldn't 
mind using PTT control but I'm not sure N1MM with MMTTY are up to the 
task.  HRD and DM780 aren't really contest software since no dupe 
checking is available.

73-

Nick, WA5BDU