overdub recording thru headphones, live track much louder than backing

If I remember reading that the delta's don't have controls or a headphone system built in like most interfaces. So you have to run the output into a mixer ( the Mackie) and then do your monitor blending there. 
 
Sonars mixer has no effect on input, that is always controlled by your interface or in your case the Mackie. The Delta needs a hardware mixer and you have what you need. 
 
Without looking at an owner manual I will make a guess that the Mackie has lots of options for the headphone monitoring. Cue Solo buttons on channels, Aux etc. 
An example of what might work:
 
Your singers mike is in channel 1
You should be using the direct output to record the overdub via the delta. 
If the board does not have direct outs then use an Aux send. You could use the main outs but then will stand a chance of having crosstalk from the playback leak into the vocal track. 
The playback output from the Delta of the computer should be running into say stereo Channel 12/13. 
 
Set your channel 1 output so you optimize your recording levels. If your using the direct outs the channel input  Gain level is where you should make this adjustment.  The channel fader may have no effect on the output level. Depends on the board. 
  
 
You should be able to blend and adjust the monitor levels now.  
 
Headphones should also be able to monitor your aux sends, so you can also set up a custom mix with that as well. I can think of lots of ways without even seeing the board. Seeing the board would even add more. 

that confused me, honestly. but thanks for trying.
 
I'm using the direct outs of channels 1-8 to go into the line ins to the Delta 1010LT. Then the line out from the Delta goes back into the mixer -  I'm using the tape input, which is set to main mix and it also goes to the headphones. I've also tried using the line out from the Delta, to the channels 12-13 of the mixer, but it the same result. Nothing I do will turn down the volume of the recorded track in the headphone, unless I send the Delta a weaker signal... which results in a weaker recorded track. I dont understand how to use a control room phones matrix, I dont have the VLZ3, I have the VLZ... I wonder if the two are different.