Perhaps the keyboard does invoke a special interrupt, but it's possible to fake the keys combination, cause obviously windows still responds to Hotkey combinations sent by PostMessage function, it's the security level that gives the trouble.
As you can see in RealVNC source code (it's opensource by the way), the piece of code that simulates the keys combination is just a regular PostMessage and WM_HOTKEY message, e.g:
' Fake a hotkey event to any windows we find there.... :(
' Winlogon uses hotkeys to trap Ctrl-Alt-Del...
PostMessage HWND_BROADCAST, WM_HOTKEY, 0, MakeLong(MOD_ALT Or MOD_CONTROL, VK_DELETE)
Either way, to use interrupt or postmessage, both can not be implemented in pure VB.