His AC should still be above 10, even without the unarmoured trait, because the normal calculation should be 10 + Dex mod, wearing armour only modifies this calculation. As far as I know, the graceful cloth is literally clothing, so it shouldn't be interacting with that calculation in any way other than the extra Dex it's giving you (which should give you +1 to your AC normally).
I thought Astarion's Con was 14, which should be a +2 mod, meaning if you multiclassed into Barb and got unarmoured defence, his AC would go up by 2, assuming you're still only wearing clothing, but no actual armour. If that didn't happen, something isn't calculating correctly.
EDIT: mentioned it to a friend and they said to check your boots and gloves, because some of them technically count as armour, and will stop unarmoured defense from applying.
Last edited by Piff; 16/08/23 12:30 PM.