My guess is if the ecu doesn't already have a common ground wire feeding it, then it would be the ecu box itself acting as ground supply. Check the resistance between it and the chassis.
The vvti requires a +ve signal, a poor GND return line will create problems. Without knowing the ecu schematics i would guess they were on the +ve side.
A simple upgrade in general is to put larger ground cables on everything.
Hope this helps.