VTEC will not engage untill the engine is up to full running temperature. It's programed into the engine management computer.

Reason being is when VTEC engages, it requires high oil pressure in the head. When an engine is cold, the oil hasn't warmed or fully circulated. If the computer allowed the cams to shift when cold and the oil not fully circulated, you'd be looking at major damage.

DO NOT attempt to rev engine over 3000 RPM untill the temp gauge is at the half way marker. Even if VTEC isn't engaging, you can still rev the engine, and that's not good when it's cold.

I always shift under 3k when the temp gauge is below the half way point.