This is a simple VST plugin by Tobybear using the formant filter algorithm from Alex (alex @ smartelectronix).

VowelSynth is a VSTi that reacts to MIDI input by generating a (non-bandlimited) waveform (saw, square or noise) of the appropriate frequency and then filters this waveform with Alex' formant filter. Simple envelope and glide control is also included. VowelFilter is the effect version of this plugin which processes the audio input with the formant filter. If you have a host that can send MIDI notes to effect plugins, you can also use the effect plugin just like the regular synth: as soon as a MIDI note is received, the appropriate waveform will be generated.