Is there a label on the ECU? Whats the ECU model code?
Possibly wiring not done correctly.
There are a number of reasons the vtec wont engage. Following affect vtec:
speed sensor not working
RPM meter not working
not enough oil pressure
Temperature sensor not working
VTEC solenoid(s) not working
How exactly is the RPM behaving? constant increase/decrease? If yes, could be a vacuum leak (or air in coolant system).
Lastly, what are the error codes?