BUR #22044 has been rejected.
mass update Alter/Ego -> vocal_synthesizer
mass update MUTA -> vocal_synthesizer
mass update NEUTRINO_(software) -> vocal_synthesizer
mass update Piapro -> vocal_synthesizer
mass update Synthesizer_V -> vocal_synthesizer
mass update UTAU -> vocal_synthesizer
mass update VOCALINA -> vocal_synthesizer
mass update VOCALOID -> vocal_synthesizer
mass update VOISONA -> vocal_synthesizer
mass update MAIDLOID -> vocal_synthesizer
mass update CeVIO -> vocal_synthesizer
mass update A.I._VOICE -> vocal_synthesizer
mass update Gynoid_Talk -> vocal_synthesizer
mass update TalkEx -> vocal_synthesizer
mass update VOICEROID -> vocal_synthesizer
mass update VOICEVOX -> vocal_synthesizer
mass update VOICEPEAK -> vocal_synthesizer
The divide between "characters for vocal synthesizer A" and "characters for vocal synthesizer B" is no longer these days. Hatsune Miku and Kasane Teto, poster girls of Vocaloid and Utau respectively, nowadays each have voicebanks in different systems, those being Piapro Studio and SynthesizerV, respectively. Yuzuki Yukari's wiki mentions she has voicebanks in FOUR systems. All this means is that it's getting harder and harder to discern what synthesizer a given song uses, and for illustrations, what exactly the artist intended to depict. Is post #6996703 UTAU Teto or SynthV Teto?
Additionally, we had songs mixing and matching characters from different engines for a long time, too. An example of such song is Triple Baka, which uses a Vocaloid and an UTAU together. Pinocchio-P commonly uses Yukkuri/AquesTalk Josei 1, an UTAU/SofTalk synthesizer, for backing vocals in many of his songs.
This tag would be similar to virtual youtuber in use, unifying characters (and most likely songs) from different companies under one tag. This way, one would not have to do an OR search with 18 tags (which first, is a pain to type, second, is only available to Platinum+ users, third, might easily time out), but rather simply search for the vocal_synthesizer tag.
Updated