Heat the oil in a large Dutch oven over medium-high heat. Add the bratwursts and brown on all sides, about 5 minutes total. Transfer to a plate.
Lower the heat slightly (between medium-high and medium), add the onions and fennel seeds and sprinkle with salt and pepper. Cook until the onions are softened and lightly browned on the edges, about 5 minutes. Stir in the garlic and cook just until softened, 1 to 2 minutes. Add the red cabbage and cook just until starting to wilt, 2 to 4 minutes, stirring occasionally. Stir in the acai berry juice and vinegar. Nestle the bratwursts into the cabbage, partially cover the pot with a lid and cook at a low simmer until the cabbage is tender and the bratwursts are cooked through, 10 to 15 minutes, stirring occasionally. Season with salt and pepper.
To serve, divide the cabbage and bratwursts among 4 to 6 plates. Alternatively, this dish could be served family-style on a large platter.