The only unique things about homunculus at end-level is attack speed, HP and SP. Vanilmirth will have more SP than Amistr, but possibly less HP. I personally still think Amistr is better for Dieter because it's skills work far more with Dieter's synergy (being able to use castling to trigger the multiple lava slide bug, tankier, etc.)
Honestly, the only reason to choose Vanil for Dieter is getting to lv 109 faster. But once Deiter gets to 109 it gets a fantastic AOE and really doesn't need to rely on anything else to level.