Players are opinions, but I think you are giving Keller way too much credit
1. Keller has a grand total of 60 games under his belt, Vicario has played 290+ despite being only 2 years older
2. Pressure of being a backup keeper and the #1 is very different
3. Keller is playing behind VVD and a midfield and system that does a lot to protect him
Keepers are funny to evaluate, most have a decent run early in their career (Pickford, Pope, Henderson, etc.) and later drop to more of a journeyman level, who knows if Keller will step up (he absolutely may).