When I'm struggling to feel out a melody for a backing track it usually helps me to play little riffs on a piano and then use that for inspiration for the vocals

There's just something natural about transferring piano to vox , it gives you a good feel for the energy and timing that's needed and leads to more tuneful melodies in my experience (works with a guitar too but it's harder to pull off)

I don't think one is better than the other in regards to doing vocal or instrumental melody first as if both are present they should compliment each other and be designed with that in mind. I reckon one catchy, well placed melody will always beat a bunch of melodies struggling against each other so less is usually more unless you've for a really solid vision for something complicated