VLS is the lowest SELECTABLE speed with the autothrust engaged. It is not the Alpha Floor or Alpha Prot speed. On the approach, S and F speeds are used by the autoflight system as target/minimum speeds for flap 1 and Flap 3 config. The aircraft can be safely flown below this speed all the way down to the Vref for your current configuration but you may encounter turbulence which could lower the buffet margin.
Operating below S speed in config 1 is not in itself dangerous and is often used by European and US operators to fly 180 when requested by ATC. However, many operators do not allow you to do this and use S speed as the minimum for Config 1. Use whatever your company SOPs say. If they are silent on this point it is probably because the management has decided to leave it up to the pilot's discretion.
On takeoff/goaround F and S speeds are the minimum speeds for retracting flaps to the next appropriate setting.