VTEC is controlled by the ECU. You could just put 12v to the VTEC Solenoid but that won't do you any good; You must be able to change Fuel and Ignition Maps under the new conditions VTEC creates. Best thing to do is to convert your car from OBD-0 (88-91) to OBD-1 (92-95). You can run either the P28 ECU (USDM Civic Si/EX) or the P08 ECU (JDM Civic VTi - This is what the JDM D15B VTEC motor came out of).
P28 - 4 wire Oxygen Sensor Conversion; Runs motor okay, Built in PA Sensor, Emmissions controls
P08 - Reuse 1 wire Oxygen Sensor; Runs motor best (meant for it), No PA Sensor, No Emmissions controls
Email/PM me with any questions. I've done this swap a few times.
BTW, I purchased my JDM D15B VTEC from HMotorsOnline.com for $495 a few years back. Freight Shipping from CA (his location) to NC (mine) made it about $700 or so. Great motor for the EF. Easiest swap for what you get IMO.