It's a bit more complicated than just adding a few wires for VTEC. Because of the wierd ass top-mounted throttle body your car has, various plugs and wires will have to be modified and/or extended to make it work.
If you can easily get the Si engine harness, I would go ahead and do that. It will make things much easier for you. Otherwise, be prepared for some re-wiring (or, in case you suck at wiring, be prepared to pay Rywire to fab up a harness for you).