set_focus_vadjustment() sets a vertical instance of a GtkAdjustment to have focus. In practice, this means that the adjustment will respond to changes made by the user, e.g. the position of a vertical scrollbar's handle in a GtkScrolledWindow will reflect the current focus within the scrolled window, when called from the scrolled window's container.

There is a method named get_vadjustment() which is implemented in all the classes where this method is likely to be utilised. Use it to provide the parameter.