Nice work.. The human voice is perhaps ...the most complex musical instrument in existence. A really well trained singer, can introduce color in/out at will.. fade vibratos in/out, and control the speed of the vibrato (most singers only have one speed of vibrato.. This level of control, can greatly enhance your composition.. Of course singers of that calibre, are going to want a fair amount of $$$ to sing for you..
Never cared much for vocal libraries, way too static.. but it's what most of us must defer do, because of recording cost, accommodation of several singers. I had one excellent untrained singer,, I often had to rewrite lyrics/melody to get the best performance out of him.. That is an important aspect a composer must take into account.. You obviously want to make the singer sound the best he can be.. Yes, learning all you can about the human voice as an instrument, can only benefit your compositional process.
Years ago, I had a singer friend, who was singing in Bernstein's 'Mass".. He was having vocal node problems, and was going to resign, cause he couldn't perform his part the way it was written.. Leonard Bernstein, was gracious enough to rewrite his part, so that he could sing it, and stay in the company.. Now thats the sign of a great composer..
Keep up the good work