To decide when you want VTEC to kick in, dyno the car with VTEC set really high, then dyno it with the VTEC point set really low. Set the VTEC crossover where the two torque curves cross for the smoothest transition.
What are you driving? Accord f22 v-tec kicks in at 2200 rpm for example and works in a completely different way than say a b16. usually factory is best because without forced induction the car is not moving enough air to use the big cam lobes well until higher rpm's. Of course frankensteins are a different breed since they use bottom ends that never had v-tec. Then it's definately a dyno job.