Sure thing. The easiest way to work things for both Karaoke and Guitar would be to hook up as I suggested above, with a couple of tweaks for the Karaoke.
I'd run an Aux send from the Karaoke channel on the mixer to the VLT guitar in. Then just turn up the Aux level on the mixer so there's some level of the music entering the VLT. If you're using stereo for the Karaoke, just send a little Aux from each of the 2 channels. To make sure you don't actually get the Karaoke track in your vocal mix, just plug a guitar cable into the Thru jack, but don't connect it to anything on the other end. Doing it that way defeats any input from the Guitar IN and only uses the information for processing key/scale for Harmony and HardTune.
With things set up that way, the Karaoke track controls the harmonies, but you'll still have independent control over the track and vocal levels at the mixer.
PS, if you want to play guitar along with the Karaoke, you'd need to choose either the Karaoke tracks to send to the Guitar IN, or your actual guitar. If you're going to be playing chords, the guitar is a good choice, but if you want to wail solos over the Karaoke mix, I'd use the track to guide the harmony, since solos etc. don't contain enough chordal information to determine a key. That was a long sentence, phew.