What OF said. :-)
Especially #2. The other weekend, a woman at the gig I was doing sound for said when we were setting up, "Please can you make sure that the vocals are louder than the piano?" I laughed and said that I tried to make that happen, and she laughed too. But thinking about it, the fact that she felt she needed to mention it is probably a clue to the quality of sound engineer she's worked with. And if you don't have a soundman and your vocals are just going through a little amp for sound reinforcement, station someone trustworthy in the audience to tell you to turn it up/down.
In defence of soundmen, some rooms are acoustically evil, so the soundman just can't turn the vocals up far enough without feedback. This is more likely when you're trying to get the vocals audible past a full band though - rarely in a folk venue.