I'm still confused...
Seems to me that the _vf to _vf_quantity relationship is many-to-one. So I don't understand why you'd need the _vf_quantity to have a key back to the _vf table, unless it were an entirely different _vf from the one already keying to it but that doesn't seem to be the case from your description. (If it is instead many-to-many you'd need a separate mapping table but you already know that which is why I'm confused. :)
second, what kind of state is selected_q maintaining vs default_q? Is this like the "user default" and "factory default?" if so selected_q should probably be in a table keyed off user to provide more flexibility. But if you really want one global value it is fine as it is.
