The VTEC's effect is clearly visible in the torque curves of the applicable cars. Whether they use the correct switching logic, I doubt; it's probably just the torque curve from a straight pull on a dynamometer, so it always "switches" at the same point, ascending or descending, regardless of temperatures (which aren't modeled anyway) and oil pressure etc.
With sample-based engine sound synthesis, you need an abrupt transition between two samples at roughly the same engine speed to get the VTEC crossover sound change right.
GT5 doesn't really use enough samples to do that.
It's quite a complex shift in sound, too. The exhaust gets cleaner and sharper, the intake much louder and slightly duller / deeper overall. But, the interactions with the exhaust tract change, as well, and more so with the intake tract. In the latter case, the increased volume excites the intake tract more (in typical applications) and it actually sounds brighter, more metallic and generally much more aggressive as a result, despite the change in the base sound being the other way.
Bear in mind that the only thing that actually changes are the cam profiles; so lift and timing. It's quite fun to try to recreate the "VTEC sound" using only that information.
It'd be nice if a sound system could accurately reflect such changes; think about the potential for tuning: put some crazy cams in your car? Yep, sounds like it, too!
One can dream...
