Combine the oil, vinegar, sugar, soy sauce, rosemary, garlic, and red pepper in a large zip-top freezer bag. Season to taste with black pepper.
Add the steak tips. Seal the bag and give it a good shake to coat the steak with marinade. Refrigerate for 6-24 hours, turning occasionally.
Remove the steak from the marinade and pat dry. Discard any leftover marinade.
Heat 1 Tablespoon oil in a large skillet set over high heat. Add the steak in a single layer and cook 5-7 minutes, until the outside is charred and the center is cooked to your liking. Transfer the steak tips to a plate and tent with foil to keep the steak warm while it rests.
Add the squash to the skillet and cook, stirring occasionally, until tender and lightly browned - about 15 minutes. Add the Brussels sprouts and cook 5 minutes, until bright green and beginning to soften. Season to taste with salt, pepper, and chili flakes. Serve immediately.