This video show how to core cabbage. However, I prefer to use a small pairing knife, rather than a large chef's knife, to do the job.
You can add the kielbasa to either the cabbage or the potato sheet-pan. I've learned that I prefer to add the sausage to the potatoes, because the potatoes soak up the flavor and oil from the kielbasa really well. The cabbage tastes more fresh, tender and crispy around the edges when it's roasted alone.
Don't use too much olive oil to roast the potatoes and cabbage, especially if you're planning to serve the dill vinaigrette. Otherwise, the meal will get too oily.
If you like your kielbasa less roasted and crispy around the edges, then roast it for only 15 minutes in the oven. You can also leave the kielbasa whole instead of slicing it, which will keep it softer and juicier.