Every time I add a melody with a distinct rhythm (from say the piano) to my vocal melody, I notice that the music starts sounding confusing and irritating. Some songs have riffs (ie a melody of an instrument) and have a vocal melody. Examples to these are Payphone by Maroon 5 and Shape of You by Ed Sheeran.
In Shape of You, there's a distinct melody of an instrument, and there's the vocal melody. I don't know if there are any rules to follow when I try to make a vocal melody sound good with the melody of an instrument.
Could someone explain to me?