I recently auditioned the KN6000 in a local music store and found that its' built-in vocal harmonizer has a 'very limited' fixed (non-editable) set (3 to 4) of presets and was of mediocre sound quality.
I use a DigiTech brand 'Vocalist Workstation EX' with my KN5000 keyboard and find it vastly superior in all areas, including: sound quality, range of presets and settings, and harmonizer & EFX tweakability.
Since you already own the Digitech VHM5 I think you will be very disappointed with the sound quality and limited features of the built-in vocal harmonizer included in the KN6000.
I had originally contemplated upgrading from the KN5000 to the KN6000 thinking that it would be great to have the vocalizer built-in, but after auditioning it, I realized that the limited sound quality & features of the built-in vocalizer is 'no match' to the Digitech units. In addition, because of the numerous reports of 'chord recognition' bugs in the KN6000, I have decided to 'hold off' even considering upgrading. I am even more concerned over reports read that Technics doesn't even acknowledge the problem exists in the first place.
I certainly am continuing to enjoy my KN5000 (composing, arranging and performing) very much. Perhaps I'll wait till the next generation of KN board is released. We'll see.
Happy Arranger Keyboarding,