A vocal harmonizer is only as accurate as its' input...

If you are looking to 'tweak' your primary vocal rather than put fake sounding harmonies around what you ARE singing, possible an Antares AutoTune AVP-1 rack unit might be the ticket?
http://www.guitarcenter.com/Antares-AVP-Vocal-Producer-101583821-i1154979.gc?source=4LANWXX1

Pretty much industry standard pitch correction, you hear it (or often don't!) on just about every CD out these days...