Because defending can net you a round win, currently there's no incentive to capping the breaker. Sit back, let you opponent take it. Defending your vault position is much easier and takes less coordination. This is a problem. My immediate suggestion would be capping the breaker nets 1 pt, a successful vault defence nets 0 pt, but a successful vault attack nets 1 pt. This would incentivize taking the breaker and attacking.