ive only ever done one original vocal track where I had an instrumental pretty much done and I sent it to a vocalist who recorded over the track then sent me the vocal stems. I went in and mixed the vocal and done my final mix.

Worked well that way because the vocalist done a lot of the heavy lifting and I already had an instrumental in place. For people who are musically trained, in a classical sense, they are good at just putting down some chords and building vocals around it then transforming it into a track.

for someone who isn't classically trained it is usually easier to have an instrumental and build a vocal around it, however; this does create some limitations I guess. For me, its really hard to write music to go with vocals, its usually easier to get vocals for an existing piece of music.

I like to get something basic down before writing vocals, so I can have some idea of where the track is going and what kind of vocals will fit.  I generally let the vibe of the production dictate the vibe of the vocals, but you also don't want to overproduce the song to the point where you can't fit them in.  If you're going to to do that, you have to stay conscious of leaving sonic space and not writing melodies over every part of the song.