Whisk the sugar and vinegar in a large bowl until the sugar dissolves, then whisk in 2 tablespoons vegetable oil. Add the coleslaw mix and herbs, season with salt and pepper and toss.
Heat the remaining 1 tablespoon vegetable oil in a large nonstick skillet over medium-high heat. Add the kielbasa and cook until browned on both sides, 2 minutes; transfer to a paper towel-lined plate.
Spread the mustard on the bread, then sandwich with half of the cheese, the kielbasa, jalapeno and the remaining cheese.
Wipe out the skillet; add 1/2 tablespoon butter and melt over medium heat. Add 2 sandwiches and press another large skillet on top. Cook, pressing gently, until the bread is golden brown and the cheese melts, 4 to 5 minutes per side, adding another 1/2 tablespoon butter to the skillet when you flip. Transfer to a plate. Repeat with the remaining sandwiches. Serve with the coleslaw.