Well this might be controversial, but I find it easier to learn the vox first, then learn the bassline. If you know the vox first you tend to have them in your head as you learn the bassline, and when you've learned the bassline the vox come a bit more second nature. If I learn the vox after, I find I have to re-learn how to play the bassline again *with* the vox.
Subtle difference, but for me, it makes *all* the difference.
I love it when you nail a vocal line that is counter intuitive to the bassline. It's very rewarding.
I did lead vocals for a while, but because my voice is good-average, rather than great, I'd rather use my voice as a great backing vocalist. It's a real strength for a bassist to have great backing vox.
I've never been short of work when I wanted it anyway.
Andy