“You can connect to “notify::position” signal.”
How to be informed when gtk.paned slider’s position changed?

“That’s the notify signal of GObject. You can use it to be notified when the property of a gobject changes.”
Re: VPaned move events

GObject Documentation

The GObject “notify” signal
g_object_notify()
GParamSpec / struct GParamSpec
gtk_container_child_notify()

Signal Callback

GObject Brain Dump