hmmm... why dont you come up with a simple lead to your chord progression and then use it for vocal.. I mean that way you know that the melody works... and then maybe fits the words round the melody...
There are some great songs which just dont bother trying to match vocals with the melody too much... have a listen to "pay no mind" by beck... great little song, sounds odd.. but works...
My best advice is simply to experiment... cos songwriting is possibley the most unique aspect of being a musician... so find your own style..
good luck
