A. No. VOCALOID is the name and trademark for the singing synthesizer commercially developed by YAMAHA Corporation. Though each particular voicebank such as Hatsune Miku based on the synthesizer is called VOCALOID too, Teto is not the case. Teto is based on another singing synthesizer and so she is not a VOCALOID.
Q. What technology is Teto's singing based on?
A. Technically, Teto is just a voicebank made for a generic singing synthesizer named UTAU. UTAU was developed by an individual developer Ameya/Ayame as a donationware and it is unrelated to Teto by itself. Teto sings by installing her voicebank to UTAU as an add-on.
Q. Who created Teto?
A. A non-profit and just-for-fun group of individuals who love her. ※要追加説明