I have not taken a "volunteer vacation," but since I am an anthropologist I have traveled in order to do research and work with local populations abroad... so it's somewhat similar. And we get paid to do it, with grants from the gov't.

I got to go to Zambia and camp out in a village for 7 weeks, very rural... no tourist would ever have the same experience that I did. I wish more people could experience that kind of thing, but I think most people prefer to have running water on vacation.
EDIT: Have you considered the Peace Corps? That's volunteer traveling if I've ever heard of it... but it involves a 2 year commitment. I have heard nothing but good things from people I know who did this.