He made game changing plays so many times. While at BYU, KVN was on a completely different tier than Fred. Some of that may have also been KVN playing in a Bronco defense versus Fred playing in a Tuiaki defense.
In the pros, Fred has obviously been substantially better. But BYU Fred is not a top 25 BYU defensive player of all time, maybe not even top 50. KVN is top 3 along with Jason Buck and Rob Morris. I'd probably go Jason Buck at number 1 and then a toss up between KVN and Rob Morris.