The Machine Simulation in NX can be set up as G-code driven simulation, which means that it takes as input the posted code that will actually go to the machine tool. So yes, G code from any CAM system can be used to drive the Simulation in NX CAM.
The Virtual NC Kernal (VNCK) is a particular optional component of Machine Simulation, with which it is possible to plug in the actual Sinumerik controller software as a black box machine driver. Machine tool builders sometimes use this capability to create very accurate simulations, even including the Controller interface. VNCK is not required for G code-driven machine tool simulation and is not typically used by most customers. It is typically used by machine tool builders, as described.