Heat the oil in a large nonstick skillet over medium-high heat. Add sausage, and cook for 7 minutes or until lightly browned, stirring occasionally. Add garlic and fennel to pan; cook 30 seconds, stirring constantly. Stir in bell pepper stir-fry and marinara sauce, and bring to a boil. Reduce heat, and simmer 5 minutes. Stir in vinegar.
Spoon 1 cup sausage mixture into each hoagie roll.