The difference between using a Kontakt Piano Instrument and a VST piano is that Kontakt is a sampler, which means Kontakt Library manufacturers can create digital instruments by using audio they've recorded (for example, a Kontakt Piano instrument manufacturer would sample the entire note on a physical keyboard for a different level of velocity qualities of attack, and more advance effects to get a similar tone of the real piano instrument, so, in most case, you won't notice much difference between them).
