Here's an unpopular opinion. "Vocalsynth" is a lame word. Lowercase "vocaloid" is much better sounding, and genericized trademarks are nothing new. Yamaha also holds the trademark to「ボカロ」btw so this is not at all different to using the word "vocaloid" in English, it's just more common to shorten words in this way in Japanese. I wouldn't be opposed to using the word "vocalo" either, it sounds pretty sweet as well. Anything but "vocalsynth".The Japanese fandom and the English fandom don't have to be the same. Japanese people use vocalo (NOT Vocaloid) to mean vocalsynth, so they *are* making a distinction. We have an English word for vocalsynth, we can use it.
I'm convinced the word "vocalsynth" was invented by the Big English Speaking Community as a way to continue selling Teto Is Not A Vocaloid to new fans.