This thread has survived some time lol.

I agree with the people who said either or works. Music first would be the way to go if you want to build upon a sound for the vocal melodies.

However, you can write the lyrics first to give yourself a topic to base the sound of your music off of. The only issue with this approach is that you will probably come up with music better than you expected and will have to completely rearrange the lyrics to fit.

Of course, it goes much deeper than that, but that's just a small portion I'm able to explain lol.