The A321 calculates Vapp differently to the A320. I meant to check the manuals last time I worked but forgot so this is from memory and should be treated as such!
A320 Vapp = Vls + 5 + 1/3 Headwind
A321 Vapp = Vls + 1/3 Headwind (minimum 5kts)
The A321 will add 5kts or 1/3 of the headwind which ever is greater to Vls. There may be a 15 kt max addition but I'm really clutching at straws now
