You've resurrected the age-old question, oh no.
Is Kasane Teto a Vocaloid? Well, by technicality, no she is not. Kasane Teto does not, as of the date of posting this, have any publicly released voicebank designed for Vocaloid, the software. While there was a Vocaloid2 version in production at one point, no official voicebank has been released. By that logic, Kasane Teto is not a Vocaloid.
However, over the years, Vocaloid (the name) has become synonymous for any vocal synthesis program. So if we take that version of the term Vocaloid, then yes, Kasane Teto is a Vocaloid. She has voicebanks for two, technically three vocal synthesis softwares (UTAU, Synthesizer V, and Voicepeak). And if we're being even more specific and defining Vocaloid as a singing vocal synthesis program that requires a payment to use, then Kasane Teto still is a Vocaloid, due to her Synthesizer V voicebanks.
Personally? Due to her SynthV voicebank's popularity becoming known far and wide across the vocal synth fandom and being one of the biggest entry points for new fans, I would consider her to be a Vocaloid in spirit at least. She's become more popular than some of the Crypton 6 (I personally knew Teto before I even knew Rin and Len), so I think that's a pretty good measure of her influence.
