Hi John
The easiest way to think of a VST is as a Hardware Expander (Like the Kurtzweils you mentioned) but done entirely in software on a computer.
As an example, take the Pocket Calculator which has a hardware case and buttons, then have a look at the calculator on your computer, both do the same job, but the one on the computer is purely software.
The software route is the most probable future of Arranger Keyboards, and I am fortunate in that I play a Wersi Abacus keyboard which has the computer built, so I can run all the software instruments I need, with no wires or configuration to worry about. (Ok there is the mains lead)
Hope this helps.

Bill