Indeed, Buga is crit immune so sadly no extra dmg. But note also that you do not need 'confirm' critical hits in 5th edition. So second Nat 20 could go for your hoof attack (or if you decide action surge now you could attack again with rapier in this same round and have that have nat 20 as well). Also for future reference if get crit the flaming chain's dmg would be multiplied as well (as I consider it a smite-like feature that occurs only during a hit and so critical hit multiplies the dmg of the chain).