I especially love vocals which are not sung, like you can find often some in older tracks, or goa/psy trance, and also in some newer trance tracks too.
something like :
Astral Projection - Let there be Light
Astral Projection - People can fly
Yves Deruyter - The Rebel
Emanuel Top - Turkish Basar
Legend B - Lost in Love
Mark Sherry - A Star within a Star
Mr. Sam vs. Human Resource - Dominator
that are the few that come in my mind ..
I think most of these vocals work more than an extra instrument, and not as vocal. They give the track an extra taste oO dont know how to describe it maybe someone understands
But I also like some Normal Vocals too, especially from Justine Suissa.
In general i think the music should be the important thing, not the vocals, and so i say the lyrics doesnt have to go about anything. they can be (and they are most time) more or less senceless, i think