If I were to transcribe my vocal melodies into sheet music, they would by necessity be 'dumbed down'- by which I mean roughly approximated into half, quarter, eighth, triplet and dotted notes for ease of reading. But the reality is that (my) vocal phrasing is far more complex than that. But notating it exactly as sung would be an extremely tedious exercise in counting notes and rests that would be pretty meaningless to another singer.
So if you're planning on notating a melody for a singer, here's what I'd do. Write it in whole, half, eighth, and dotted notes (16ths if needed), but also make an audio file of how you imagine the interpretation of the melody to be. Every singer has his or her own quirks which inevitably find their way into a performance, and by rights belong there too. Take away too much of a singers personality and you end up with a rigid, lifeless performance.
When you're actually writing the melody, the last thing I would advise having in your mind would be sticking to any idea of note values. That's something to work out later.
One last word: learning theory is great, but it's 100% better when learned in conjunction with an instrument. Theory without practise is, well, just theory!