Think of the under-mixed vox as a mercy killing.
I've been known to dip the lead vocal at points I knew the singer wasn't going to hit the note. As long as you can hear the vocal most of the time, I don't even have to take the blame.
One tech who worked for me took it a step further, on a week long gig in a club with a girl singer who could barely carry a tune in a bucket. After the first night of enduring her "pitchiness", he plugged in another mic at the console and sang the parts (in falsetto) that she couldn't. The rest of the band was in on it. I don't think she was any the wiser.
He has since moved on to bigger and better things than working in dirty old nightclubs:
http://www.rkrecording.com/Now he spends his time polishing symphony orchestras that cost hundreds of dollars a minute to record.
GTD