We currently have quite many tags for various pieces of vocal synthesizing software - they can be seen in the list_of_vocal_synthesizers. However, we have no real way to search for characters associated with the software itself - we have copytags like Vocaloid, but the problem with them is that:
1) They only cover one specific software,
2) Original characters created for songs using this technology are also covered by those software tags (think of Aimaina or Osage (Inabakumori), who are not Vocaloids themselves, but are present in many songs by their authors),
2a) Similarly, characters in costumes that reference a song's PV also get those tags - see post #6219029, which features Yu Gi Oh! characters in headphones from Magnet.
3) Nowadays, many characters have voicebanks in more than one specific synthesizer - Hatsune Miku, the poster girl of Vocaloid, now is only developed as a Piapro Studio entity, and Kasane Teto, one of the most popular UTAU characters, recently got a series of SynthV voicebanks - which means the copyright the character belongs to can be muddied.
Additionally, subcultures of such software often merge - from earliest days, we got songs like Triple Baka, which featured a Vocaloid and an UTAU, covers of songs from one software to another, or shared art like post #5032611.
The tag would function similarly to Virtual YouTuber.