I don't sing, but I've studied arranger behavior for 13 years. At least up through the Tyros 5, Yamahas have what I would call "hair trigger" chord recognition. Depending on what chord input mode you use (multi finger, AI fingered, etc.), Yamaha may prematurely acknowledge a chord as soon as the first key is pressed, and then change it once all the notes are down steady. [If you want to see this for yourself, do a quick record that contains a lot of fast chord progressions, then play it back at a much slower tempo. Watch the arranger fumble around changing its choices in the "Chord" area of the main display. The recorded midi will also be loaded with portamento control events.]

If that same logic is driving Yamaha's vocal harmony voices, I can see why they would sound slurred, or also have a portamento effect.

IMO Korg has the most decisive chord recognition, although you have to "lead" the arranger a little bit to make sure the chord is recognized on the beat. That could be why everyone says Korg's VH is the best.