MSD makes a RPM activated switch and has all the instructions to wire it up, but I would suggest converting to OBD 1 and getting the P08 ECU instead. VTEC works off oil pressure. A RPM switch will not read that, so if for some reason you run low on oil or pressure, you have the chance of seizing your engine, where as, the ecu will not let you go into VTEC if it detects low oil/pressure.
matt