<<Try looking at the documentation for WM_VSCROLL again. It says:
LOWORD(wParam) = SB_THUMBTRACK
HIWORD(wParam) = thumb position
lParam = HWND of the scrollbar control, or NULL if not applicable
Note that WPARAM is a 32-bit type just like LPARAM.
Many apologies for the bandwidth, I stupidly was using a Win 3.1 API reference book. I assumed wrongly that simple stuff like the scroll parameters would be the same in Win32. Undoubtedly this will explain some of my other difficulties.