The really great thing about vtec controllers is that they change the vtec engagement RPM, but your ECU is still doing whatever it wants. So depending on whether you lower or raise the RPM setpoint, the ECU will still be on the low cam maps, or will have already switched maps when the vtec controller does it's thing. Not a good idea. They are, however, a good idea if you do a LS/Vtec swap and need something to activate vtec. That assumes that you're using your LS ECU and have some other way to do fuel tuning.

 

What he said, although there are some instances when simply lowering your lo>hi crossover is a good thing.

I will add that if you MUST lower your VTEC point, you don't have to lower it much. Honda typically sets the crossover point artificially high so that you can "feel" the changeover. For most stock cammed Hondas, VTEC lo>hi should only be about 500rpms lower than what's stock. I lowered the point on my S2000 for track days just to smoothen out the powerband. Throttle on, increasing radius corners are a little easier without a sudden peak in the powerband.