Where should the vocal image be?

Usually sounds behind and midway between the speakers, mid-height too. 

Mike

In front of the front omni's with a believable spread.  But as Erik points out, program dependent.

Done it before, just upgrading my means currently.

Turn it over to the 'bots, they've nothing else to do for me.... ;)

BEHIND

Vocalists at proper scale (ie the visual auditory image is human scale or slightly larger) should be behind the plane. Exaggerated scale or vocals that are too forward indicate peaks in high-frequencies and can cause destruction of a 3d stereo image.

Agree with many here who say the recording, etc. are influential. Also, agree that test tracks are invaluable for calibrating a system. 
 

(not to get involved with the toe-in discussion, but the "mid channel" of the stereo signal (where many vocals live) often emanates from the side of drivers and LESS toe-in can increase the mid channel content, like vocals. So toe-in and speaker position definitely are very important to vocal depth.)

Personally, I prefer they be on someone else's stereo. LOL

OP I hope you found a baseline from all this???????????

I think one thing we can agree on is that there are a few variables.  Once you have an optimal setting, most likely it's the recording that dictates where the vocal image will be placed on the soundstage.