Pete of Casper Electronics posted his picks for better bending on the classic Casio VL-Tone keyboard -

Almost all of the bends in the VL-tone are achieved be creating amplifier feedback loops. There are no data glitches to be found. This means there are no random melody bends but lots of squealing distortion bends.

Seems the little VL sports a rather delicate LCD connection that requires a fair amount of TLC. Relevant tips + wiring diagram can be found over at Casper Electronics.