But one thing not immediately apparent is that the Vapp comes from the FMGC and Vls comes from the FACs. Which means that if the aircraft is a different weight form the loadsheet, (and we all know how accurate those are if you're using notional weights) you may not see the 5 knot buffer between Vls and Vapp you should on your PFD.
The 321 seeems to be the worst. In my company, two or three tons difference is the norm. I have seen as much as five. Manual insertion of Vapp is common, I tend to add one knot per tonne of difference to the FMGC generated Vapp having checked the FAC generated weight.