Why can the A320 VAPP be decreased down to VLS?
... if landing is performed without A/THR...

Then theoretically you don't need the VAPP [setting] and should fly a bit faster, it's only a speed bug in this situation.

... without wind...

Even if the METAR is reporting an average of 0 knots, there is always a breeze here and there.

... no margin against a stall...

You have a 23% margin. VLS is 1.23 VS at 1G (check FCOM 4.03.20). So if the stall is at 120 knots, then VLS is 148 knots.

Note:


  VAPP should be at least VLS +5 kt.


So VAPP even becomes 153 knots, 33 knots above stall. Plus the A320 by design has anti-stall measures if the flight laws are not degraded.