>You can send the stereo pair of outputs from your Motu to the mixer.
>Send the vocals direct out to another input on the mixer.
Right, but it won't work because the main outs from the MOTU carry all sound, that means you'll hear the vocal with the music output. I know because I just tried that.
Lowering the main, lowers the vocal as well, even when there's a separate vocal input from the MOTU. There has to be a bus in Sonar that allows you to send all but the vocal. Then it should work.
I haven't done it yet but I think msm (above) nailed it with the bus idea, that gives me two separate sources so I can affect one without affecting the other.

@Chuck >> No disrespect. You nailed it in your post.
 

revnice1
Right, but it won't work because the main outs from the MOTU carry all sound, that means you'll hear the vocal with the music output. 

 
I often have an instrument bus and a vocal bus that come together for the mains.
 
Rather than sending the mains to the mixer, send the instrument and vocal buses.
 
You can still monitor them together out of the headphone out on the Motu.
 

I've got one of these things, a now-discontinued product but I found one on eBay.

It has two inputs, one for your mic and one for the cue mix. The output contains only the mic signal. It has enough gain to blast the mic signal as loud as you want it.

I set things up the way msm suggested above, namely:
 
MOTU Output 1 & 2 (Main outputs) -> your monitors
MOTU Output 3 & 4 -> your Mix5 mixer (with your singer's headphones plugged into it)
 
I can now change the relative levels on the Mix 5 - but only on playback!
I can't find a way to monitor the Input. Echo On works but there's a very distracting delay.

I wrote to Mackie last night and they responded this morning. Support told me that what I'm trying to do is not possible with the Mix 5 (as it stands) because it doesn't have an Aux Send. The Mix 8 does, that's the one I should have bought.
I was pressed for time in Guitar Center when I bought it but the sales guy assured me it would work and I thought so too. The promise of stardom and the alluring smile of the Mix 5 was too much. 'Hang the expense,' I said, as I took the $18 plunge. I wish I'd spent $20 now!

Hey, revnice1, actually I wouldn't bother with the Mixer at all in your case.  (I originally thought you were using the mixer for all inputs and sending it's mains to the MOTU and SONAR.)  However, it is a little tricky to create a particular 'sub-mix' for the vocals out of SONAR to your MOTU and out the phones to the singer.  It's more of a mental mapping within SONAR that actual physical routing, but is totally doable as described by others previously as is just using the MOTU mixer software without SONAR's involvement.
 
However, again without the need for a mixer, what bitflipper suggested is just like I was suggesting but in reverse of the routing I described (since I thought you were using the Mix5 as your main mixer).  It's more simple and direct.
 
It is doable.
 
Kind regards,
 
tecknot

Before all this began, I contacted MOTU and they said sure, no problem and sent me instructions on how to configure the CueMix software. It's possible I didn't understand it but I remember following it word for word and I still didn't get what I was looking for. I also remember being nervous about ruining the entire config. 
 
Have you ever pressed the wrong button on the TV remote and spent the next half hour trying to get the picture back? :) I once screwed up a Kontakt installation which affected ALL my projects. I had to open each one in Safe Mode and get rid of Kontakt before adding a corrected one. In some cases, I couldn't remember which drum kit was originally loaded, or the settings, it was a disaster. So I was very wary when playing with routing and busses, etc.
 
Then I thought a physical thing with knobs on it would be much more intuitive or at least you can see where the signal is going. Two horrible little devices later (admittedly the wrong devices) and I'm going back to MOTU and the CueMix software - only this time I think I'll have someone help me live on the phone.
Once I get it right, I can make detailed notes.
 
rev

revnice1
I set things up the way msm suggested above, namely:
 
MOTU Output 1 & 2 (Main outputs) -> your monitors
MOTU Output 3 & 4 -> your Mix5 mixer (with your singer's headphones plugged into it)
 
I can now change the relative levels on the Mix 5 - but only on playback!
I can't find a way to monitor the Input. Echo On works but there's a very distracting delay.

I can think of 2 possible remedies for this:
 
1. Reduce your latency by either:
   (i) Freezing all your tracks; or
   (ii) Bounce everything down to a stereo guide track that your vocalist can sing along to, and temporarily mute or archive the other tracks.
 
2. Enable input monitoring on the MOTU - CuteMix should alow you to do this. If you're going down this method, I wouldn't bother trying to send your vocals to any extra SONAR bus - just get CuteMix to route the vocals to both your DAW and one of the MOTU outputs (which you can connect to your mixer).
 
M.

> (i) Freezing all your tracks; or
I don't understand that one, I've never frozen a track or plug in 12 years. 
 
>(ii) Bounce everything down to a stereo guide 
I often bounce down and put the result in a new project because I anticipate at least 10 tracks of vocals and with all the other tracks and plugs, it's often too much for my box. It doesn't reduce latency but it does make the project a lot easier for my machine. (Asus P6T, Intel 3.33 i975 quad core, 12 RAM)
 
I could try increasing the latency offset but I think I'm already at the max 5000
 
>just get CuteMix to route the vocals to both your DAW and one of the MOTU outputs ...
That sounds good! That would act like the Aux Send I don't have.
 
My problem now is the singer is due any minute!