I've been told that in FI you engage vtec at the point of positive manifold pressure.

I had that setup but with a OEM CAT/Exhaust and had a huge amount of soot on my bumper in a short period of time. The dyno showed a good dip in power too.

As soon as I moved to a TP, very little soot (I haven't been on the dyno, but i'm almost 100% positive that I no longer have a dip in power)

 

On an Sc generally yes, give or take a few 100 rpm, depends on the motor and how efficient your exhaust system is at that rpm range. Sometimes primary cam in boost is a bit more responsive yielding a little more trq up to a point. The only real way to maximize this engagement with your car is to get it on the dyno and see where vtec starts to pick up power over the primary cam. Its very strait forward at that point, no guessing. It should be a strait/linear line from primary cam to vtec engagement, all the way to redline, there should not be any bump up in the graph when vtec comes in, if there is, then its not low enough. If the power dips down at vtec engagment, then its too low.