Audio Circuit to Remove Vibrato while Preserving Word Detail
One relatively-regular low frequency (~2-20Hz) overlaid upon the existing "note frequency" (the 80Hz-15kHz normal range of human voice determining note in the music)


This is not entirely correct. Vibrato is not a 20Hz component summed with the vocals, it's actually frequency modulation: the LF component is modulating the vocal range.

To remove vibrato, you could try use autotune with a short response time (to stabilize frequency) and compression (to stabilize amplitude). Careful tuning of both these effects would likely do a decent job of removing vibrato. However, note that this is not something that can be done with a simple analog circuit. Pitch-shifting of complex signals such as vocals (let alone autotune) is decidedly non-trivial to implement in the analog domain.