VTEC can be SOHC or DOHC. I think all of the SOHC VTEC motors have been intake only, while with the DOHC motors some have been intake only and some are both intake and exhaust.
Some versions of VTEC change cam lift and duration with the third lobe, some shut down the use of one valve during low rpm operation (as seen in the fuel efficiency and motorcycle versions).
The performance car versions of VTEC use a mild cam profile for lower rpm/cruising mode, for best fuel efficiency and emissions, and a hotter cam profile for high rpm/WOT situations, which allows for maximum flow at high rpm.
Basically any motor which uses conventional cams is a compromise. The best cam profile for low end/low rpm performance is very different from the best cam profile for high end/high rpm performance. High rpm calls for long duration and high lift to cram as much charge as possible into the combustion chamber. The problem is, at low rpm, optimal air velocity is lost with this long duration/high lift setup. Lower rpm requires the valves to be open less and for less time to keep the charge flowing optimally.
Consider sucking air through a straw. If you suck x amount of air through a small diameter straw, it will travel nice and fast. Suck the same amount of air through a big huge honking straw, and the same amount of air will travel slowly. Engines need the air to travel fast for the mixture to swirl around and pack into the combustion chamber for best results. The problem is, the small straw will only suck so much air. You need a small straw to suck small amounts of air (low rpm), and a big straw to suck large amounts of air (high rpm). Small straw - mild cam profile. Big straw - hot cam profile. Non-VTEC motors choose a in-between straw size that suits both as best a possible, but will never suit either one optimally.
VTEC is basically two sizes of straw for one engine... That said, any manufacturers variable valve timing and lift system is basically the same principal. Variable cam timing, that is another story...