No. since the ring already gives the feat.. the actual feat does not stack. all that means now is, you're only wearing the ring for the +2 damage boost. that can only be taken if you had 4 levels of fighter.
Edit: So you'll notice if you take the ring off you'll lose the extra damage but still have 19-20 crit range. Imp crit feats don't stack.
It only showed up on leveling because you didn't have the feat naturally.