Vls is the lowest speed you are allowed to fly for given phase of flight/configuration. It will be different for each phase (eg. for takeoff Vls=V2min or 1.13*vs1g, later on it will change during flap retraction)
For approach Vls=Vref as you stated. Technically, you could do the approach at Vls, but Airbus recommends to add at least 5knots (or more if there’s significant wind). Also, 5 kt is mandatory if you use autorhrust...
You could do this when using manual thrust, but it would have little benefit in terms of landing distance. At the same time, you would increase risk of tail strike, or low energy situation hard landing etc. - so no one ever flies approaches at Vls