I tried the vocals last night.
My wife is still not feeling well and did not participate.
I plugged in Two USB Microphones, and selected harmonies rather than solo. No, I have not mastered the art of Overtone Singing, but I wanted to see how it worked—because each instrument is generally controlled by a single Wii remote. The Microphones are controlled under a single Wii remote, and score as one player. So you can play with up to Six people at once: Guitar, Bass, Drums, & Microphone 1-3 (Some songs have a single vocal track some two, but most seemed to have three).
I did not fail out for not harmonizing. I tried on the first harmony track to sing the harmony, but couldn't quite find it, but at the end of the song it told me I got 1 of 7 harmonies.
I need to get a USB Hub and another Mic and have Some people over to play. I still don't think the gameplay is as optimized for group play as Guitar Hero; but being able to play with six players is cool. I'm Sure Guitar Hero 6 will steal and Improve upon the mechanic set forth by The Beatles: Rock Band.