Currently the skill is quite bad for the amount of skillpoints invested, with its long CD time and its low uptime, unless you got a lot of vigor (at 75% vigor you got 2/3 uptime on this skill).
So how about having the skill do what the description says?
It's written that you infuse your shield with your Aura, so how about changing the skill to require Aura to cast, and add an effect similar to Concentration, increasing the lasting time by 5-10 seconds for each Aura used?
The lasting time would be 10 seconds with 1 Aura on cast, 15-20 seconds for 2 Aura and 20-30 seconds with 3 Aura consumed on cast. This would mean that the skill becomes more useful early in the games progress because you still have less Vigor and makes the Knight more tanky in the process because he can achieve 100% uptime on the damage reduction if he invested skillpoints here.
Also, looking at the other Classes skills to increase Def/decrease damage, many of them (that are single target skills) are currently way more potent with maxed Vigor (Defender +Parrying combo on Warrior, Shadow Armor on Assassin, Grizzly Form + Wild Impact+Beast Charge for perma stun, Iron Skin+ Steel Body on Monk), so given the the role of a Tank, the Knight should be able to have a good buff with enough uptime upon his sleeve which distinguishes him as such.
I really like the idea of gathering Aura and using it for attacks and Heal, so why not utilize it for defensive purposes,too?
Aura Shield seems to be the skill that's the easiest to change, + it requires only little work to be implemented as the code can be copy-pasted from Concentration and just requires adjustment regarding the uptime, thus being not susceptible to bugs.
