Dyno graphs, 2 of them.
One pull with no vtec, one pull with vtec all the time or set really low. Where the graphs overlap is the best place to set vtec. This way you'll maximize your power curve because vtec will take over when the lower lobes fall off in power.. It's very easy really.. Changing vtec rpm is not easy on an OBD2 car though, because the ECU can't be chipped and you have to rely on a piggyback computer as a band-aid for proper tuning.