But you could also use Logic's own audio region pitch transposing function to copy a vocal recording and change its pitch to become a second, third or fourth harmony. Any audio recording done in Logic should work with that. If importing a file recorded elsewhere you can re-record it in Logic to make it "pitch change enabled". Route the track's output to a bus and use that bus as the input for a new audio track where you record it. Then click the little channel strip inspector box named "follow tempo" and you can cut the regions into slices that may be pitch transposed.
The "follow tempo" method might feel bulky if there are lots of glissandi in the original recording. You may also experiment with sampling a part to be played with the EXS24 and pitch bend.
Finally there is also the option to use the EVOC20 to create vocoder sounding harmonies out of a lead vox track.
You may as well get Melodyne right away. It can do harmonies (if you can figure them out yourself, it can't do that ....yet).
It's a fantastic tool for correcting pitch and to some degree timing too.
PS When I say it's fantastic I mean that it does what it's supposed to. I would just wish that it wasn't needed quite as often as it is or maybe it's just my clients (who hopefully doesn't read this forum).
It works, everybody uses it, the expression "to autotune" is changing to "to melodyne". I like the plugin version but that may be because it's the one I have and know. It's also the one that just works as any other plugin in Logic.
Also check out MU Voice. It's a real time 4 part harmony plug-in that can sync to your seq. or can be triggered from a keyboard on midi controller. They have some videos on youtube that are pretty cool.