You need to have good counterpoint between the vocal and the instrumental line. Then you need to use your ears to write counter-melodies that complement and enhance the vocal.
How do you write a melody with vocals?
How to Write a Vocal Melody
- Follow chords. ...
- Follow a scale. ...
- Give your melodies a focal point. ...
- Write stepwise lines with a few leaps. ...
- Go outside to write. ...
- Get inspired by your favorite artists.