I don't think software is good enough to do this at this point in time. I'll probably hum and grunt my notes, drag into Melodyne to figure out what exactly I vocalized, then perform them on an instrument.

 

I find this tool very helpful - once you know how to do it, yes, it's not that easy; but you can create something

Waves - OVox Vocal ReSynthesis
- Transform your vocals with the next-generation voice-controlled synth
www.waves.com/plugins/ovox-vocal-resynthesis

Play Any Virtual Instrument Using Your Voice via OVox’s MIDI Out

OVox’s Midi Out feature lets you play any VI (by Waves or other manufacturers) using your voice, in real time.
Sing and hum a melody, and OVox will convert your to MIDI, and send the MIDI out to your instrument of choice.
You can also use the note mapper to quantize your MIDI to scale, or generate hundreds of chord progressions easily.

 

Vochlea Dubler was made for the specific workflow you are asking about, even though it still internally creates midi data. Don't take this as me recommending it. You could try a demo version.