Aux sends......
.....if I'm understanding you correctly......
you want to monitor (w headphones) the whole mix including the vocal but want to record the vocal seperately as an overdub.
Send the music mix into one of the stereo ins (either 5/6 or 7/8) and put the mic through a mono channel.
Use either Aux send 1 (pre-EQ)......my choice.....or Aux send 2 (post-EQ)....to the soundcard.
You can track in stereo this way by sending mic 1 thru Aux 1 to the left channel only of the sound card and mic 2 thru Aux 2 to the right channel only of the sound card.
Unless you do the board mod to the 802, then Aux 2 if fixed as post-EQ.
You other option would be to pan the channel the musics on all the way to one side (right) and the vocal channel all the way to the other side (left) and then monitor on the headphone out but only sent the left mix bus to the sound card.
Your better off just using the Aux sends though and you wont have to worry about and crosstalk on the mix buss.
-mike