Diarra improved throughout the year. Vogt regressed.
Diarra's issues are correctable, Vogt is stuck being slow and unathletic.
Diarra played the important minutes over Vogt in the last two games of the season, had tournament been played this trend would likely have continued.
Diarra was the better player at the end of the year, given that he also has a much higher ceiling, there is no reason to assume he won't be the better player next year as well.
As Justin says, they may well start together. But if we go small, then Diarra should be the starter at the 5 over Vogt.
But on the original subject. Vogt has not proven he deserves to be the starter next year. There's no reason to think that Brannen would be telling a grad transfer big they'd only be getting bench minutes. Vogt had the opportunity and didn't do enough to prove he should start. Diarra showed potential and started to show results, but hasn't proven himself as a starter. Eason is a freshman and may not be worthy of more than 5-10 minutes per game in his first year.
There is a lot of room for a big to come in and get 20+ minutes per game. The question is does Brannen want to go that route? Based on his recruiting I suspect he is looking at playing Diarra at the 5 and playing a small ball 4.