I was wondering how good idea can be to change vtec crossover using vtec controllers, because I understand the ECU changes drastically ignition time & fuel maps when vtec is engaged, what happens when you use a vtec controller? does the ecu know VTEC has been engaged with an external system and also changes ignition time & fuel maps??

 

After your car has been tuned to whatever crossover point the tuner set it as, whether for power, fuel economy, smoothing power curve etc., you should not mess with it after. So don't just go using a VAFC to change it unless you know what you doing.