The difference is that Vogt's deficiencies are much harder (or impossible) to fix than Diarra's. Vogt is never going to become much more athletic, much more mobile or a skilled shooter. His shooting form is not something likely to be fixed in an off season. Sure he could add a little agility, a little strength, box out a little better. But his ceiling isn't much higher than what he is now: a guy who can beat up on undersized, less athletic teams, but who can't deal with teams that have size, strength and/or athleticism. Should be a backup at this level.
Diarra's issues are due to lack of experience. He has the size, the athleticism, the mobility, the shooting ability and even some ball handling. It's just a matter of him learning to put it all together. Experience is gained through playing time and as he gets more he will just continue to improve. That's part of why his improvement has been so rapid this season. Give him starting minutes and he may be the best player on the team by the end of next season.
Diarra is already far better on defense, a better rebounder and a better jump shooter than Vogt. Diarra is much more likely to surpass Vogt on offense (if you believe the conference ORTG numbers, he already has) than Vogt is to get near Diarra's level on defense/rebounding.