"...from the vocoder, where you can just play the keys and get the source tuned to the carrier frequency?"
That's not what a vocoder actually does. A vocoder analyses the rough frequency spectrum of a (source) signal, and uses this spectrum as a kind of equalizer envelope for the processing of another (carrier) signal. If the carrier had a very even frequency spectrum before, it will thereby pretty much take over/copy the frequency distribution of the source. The source itself is not processed in any way at all, apart from being analysed.
The carrier is usually such an even-spectrum synthesizer sound, but you can use any kind of signal. For instance, you can use a string quartet and modulate it by feeding a drum set as the source signal!
On the other hand, "where you can just play the keys and get the source tuned to the carrier frequency" describes quite accurately how a harmonizer works. The problem with this is that you need to know the exact frequency of the source. It needs to be a proper clean monophonic sound, otherwise the algorithm can't know how much it needs to pitch-shift the signal to reach each of the carrier frequencies.
What's more, in a harmonizer you usually don't specify the target frequencies manually, but just pre-define some scale for the harmonizer to work on. It then needs to figure out by itself which harmonies to actually create, which again only works if you start with a very clean signal.
