In a large bowl, mix together vinegar, adobo, pepper and oregano. Add steak and onions to the oil mixture, tossing to combine. Cover beef; transfer to the refrigerator. Marinate at least 1 hr., or up to 12 hrs.
Heat oil in a large skillet over medium-high heat. Add beef, onions, and marinade to the pan; cover the pan and cook, stirring occasionally, until onions turn translucent and beef is cooked through about 10 minutes.
Evenly divide the meat among serving dishes; top with onions and drizzle with sauce.
Arroz Con Gandules:
Mix all together and bring to a boil for 3mins, stirring frequently. Then set heat to low, cover and let cook for about 45mins or so.