Maybe this is an oversimplification, perhaps even a benefit of not having had the sophisticated argumentation of some theologians, but this has always been clear to me. When the Westminster Standards use the term "obedience" it means the whole of Christ's obedience, which includes both His active and passive obedience. Because Christ obeyed God, He met God's perfect standard of righteousness, and faith in that saves us. 'Federal vision' at best, would confuse this simple truth, which undermines God's promises and our assurance. At worst, it denies it. Few things could be more damaging than that.