Nice mash up idea.Cool track. What u need is software to pitch shift the vocals to the key of your track while also matching tempo. audicity can do this,but takes trial and error, Acid is good 'cos if you 'acidize' the vocal sample you can stretch it out and it will keep its key,although you may have to transpose your track to match. Your track sounded like it was pitch effected(slightly glitchy?) It will sound more natural if you effect the vocal instead.
Other members might suggest better software to do this. I am only really familiar with ableton live software and old outboard gear.