I wrote a few songs and I have all the instruments worked out (Guitar, Bass, Drums) and I have a problem thinking of the vocal rhythm for the songs I write. What are some tips that will help me fix this problem?
well personally for vocal melodies I just start humming to it / singing random words till I kinda like whats going down. Then I'll write actual lyrics to that then put them together with the song. Once I've done that I see what can be improved and often tweak it or change stuff totally.

Thats just me. You gotta find your own way of figuring out what should be there. There's no straight way. Just what you think will sound good really.