If you want to do it the easy way...here it is: Only 3 people required...
Put all the instruments on beginner.
1 person plays bass, one on the drums, and one is the guitarist. Obviously...
Now take out a fan, and put your GH:WT microphone or Xbox 360 headset in front of it. Or use something else than a fan; as long as it makes enough noise it's fine.
Now play, and you will see that the game picks the noise of the fan up as singing. So that's a sure way to get 100% on vocals.
Now, the rest is up to you. Don't miss any of those notes!
The 'fan-method' works on any song.