did you make sure that your non-vtec oil pump is enough to keep the vtec system engaged? That may be why it's jumping on and off. You may also need to tweek your fuel curve as well.

And yes, voltage will affect the solenoid. Too little, it may not engage fully, or may sort of *hop* between engaged and disengaged. Too much voltage, you'll overheat the solenoid, possibly damaging it.