The host for the Karaoke only "runs" it. There is a small band there that plays the music and there is the Karaoke machine that has the words display. So you really don't need to do much but keep track of who wants to sing and maybe suggest songs for those that want to sing but don't have a particular song in mind. That part is easy. My advice is to listen to your crowd, let them know this is your first time running the Karaoke and let them provide suggestions as to how it should be done. Many of the cruisers are past guests and will have been to other ships shows and know how the "good ones" host the shows.
I'm not sure what a vocalist for the show band does. Unless you are in one of the smaller bands that use around the ship. The main show band only plays music I have never heard anyone except for the headliner singers actually singing along with them. Maybe one of the smaller bands (which could be 2 people, as Carnival seems to like this setup) or the casino band (3 or 4 musicians that play requests at the casino area) needed a vocalist.
I wish you luck, it sounds exciting.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013