I can get VOCALOID5 VSTi (a Yamaha voice-synthesis engine) to appear as an option in the Play synth-selection list, but so far cannot get it to recognize MIDI notes in the instrument linked to it. This does not surprise me, since VOCALOID is a very different sort of VST in the way it processes MIDI data to add synthesized lyrics (and the default voices seem to have a Japanese accent even in English–trying to get them to pronounce the word friends is pretty much hopeless 
). But VOCALOID has been designed to work with Cubase, so down the road (way down the road?) it may be possible to integrate it into Dorico too.