Well I'll be petty now and use the proper official spelling of VOCALOID now because you want to correct that.
The point is that using the official (or uppercase-first) spelling is
wrong, as the word I think is best to use for the broader category of vocal synthesizers is "vocaloid". Specifically lowercase,
because it's not the name of any specific engine.
VOCALOID™ - a specific vocal synthesizer engine made by Yamaha
vocaloid - some vocal synthesizer
Also, it's not a matter of "what word sounds the prettiest" its "what word can convey what a vocal synthesizer is the best".
Words convey meaning by existing and being used, not by what specific letters or other words they are made of.
I mean, I specifically do discuss engines. Macne Nana is a VOCALOID and UTAU because she was on both engines. If she wasn't on VOCALOID, I wouldn't call her a VOCALOID.
When discussing engines, in which way you use the word "vocaloid"/"VOCALOID™" is clear from the context.
"The recent vocaloid song by XXX featuring IA" (even if it's the CeVIO voicebank)
"The recent vocaloid song by XXX featuring IA V3" (it's definitely not the CeVIO voicebank now)
"IA recently got a new VOCALOID™ voicebank" (clearly you mean the VOCALOID™ engine, even if you are speaking out loud or write everything in lowercase)
"She's a vocaloid producer" (maybe using SynthV)
"She produces songs using VOCALOID™" (now definitely not using SynthV)
What do you think of the words "photoshop" and "google"? Is it weird and confusing if I've "photoshopped" an image with GIMP, or "googled" something outside Google? Would the word "immanipped" be better, because it combines the parts of "image" and "manipulate"?
The phrases "vocal synthesizer" and "image manipulation" are normal phrases to be used in professional contexts, but that doesn't mean "vocaloid" and "photoshop" are worse than "vocalsynth" and "immanip" in a casual context.
That said, I agree the distinction between the broad "vocalsynth"/"vocaloid" category and the subcategories of specific engines
is often useful, and I'm not trying to argue that it isn't. My point was that I don't think it's important that the
name of the broader category must be completely separate, though I guess I didn't get it across very well.