Given that it stacks, I still think it should only be +1 instead of +2 damage...
I also agree that it should probably be a bit more expensive.
Making it only apply once for adrenaline should help keep it from being OP a bit though.
Not too bad otherwise though...
One question: How will it interact with nightfall.
I.e. if I have nightfall, will this be applied before or after?
E.g. will a vampire benifit from both and become a 5/3 or only from nightfall and become a 3/3?