When I track vocals, I like to have the artist in the same room and close by. It makes it easier to communicate and the session is more intimate and personal. Nothing kills the mood like being stuck in an isolation booth and looking through a glass window.
This also makes it easier to keep an eye on their body language. It may not be obvious from another room, but a lot of fidgeting and shuffling will ruin a recording, and it could be a sign of nervousness or that they are uncomfortable. I always make sure I say something after each take and ask if they need anything (water, etc). I also like to set the mood by dimming the lights or using candles.
For this session, we recorded lead vocals, a double of the lead vocal, and background harmonies. Each vocal was approached slightly differently (vocal register, tone, distance from the mic) in order to give it some separation in the song.