Turning vocal into midi

What Zarrg71 says usually works, although sometimes the midi timing won't be correct. 
 
Here's how I've done it:
 
1. Sing in a non-fancy way -- for example, if singing a bass line, just stick to a single, simple word, for example, "BAH-bah-bah, BAh-bah-bah"  Don't worry if it sounds a bit stupid, just concentrate on the groove and the pitch. Don't use any FX on the vocal.  Sing to a click track or some other steady beat.
 
2. Open the recorded vocal wav file with Melodyne. In the small window at top right which shows you BPM when you choose the right tool (I forget which one and am not at my music PC, but it's easy to find), manually set the tempo to however many BPM you already know the click track is. (Melodyne tries to guess, but isn't always right; this step makes sure it'll all be in the right time). 
 
3. Press CTRL+a to select all "blobs"
 
3. Use the "Correct Pitch" button and dialogue on the top right , then set the Pitch Correction, while auditioning (meaning, when it's playing back) for however strong you like (100% is ok for these purposes). Also set the Pitch Drift up to 100% if desired. 
 
4. Close the Pitch Correction dialogue, and with all blobs still selected, choose the Pitch Modulation tool (this is the second subtool you'll see under the main pitch tool; the main pitch tool itself is the second from the left in the toolbar), place it over any of the selected blobs, and drag down; you should see the thin, wavy pitch line within each blob become basically a straight line. This basically flattens any vibrato in the voice.  This will make the vocal sound very artificial, but that's ok in this case; we're just making the pitch information less confusing, so that Melodyne can do a better job of translating it into midi
 
5. Under File, click Save as midi... Remember where you've saved it. Then with the browser, drag the midi file into your project.
 
It doesn't always work miracles, but can work really really well sometimes. You might get some weird, mis-translated midi notes, but you can clean those up in the piano roll view.
 
I've used it so far to control virtual flutes, of all things. No one I played the track to had any idea it wasn't a real flute (though I'm using an unsually good flight library).
 
In all, a very powerful tool for writing parts for certain instruments you can't play.