In a zip-top plastic bag or a shallow plastic container mix vinegar, sugar, Cajun seasoning, soy, oregano and salt and pepper to taste. Mix well. Add the steak, seal the container and shake well. Marinate overnight.
Preheat the grill to hot. Grill the steak 7 minutes on each side for medium. Let rest 10 minutes, then slice across the grain into 1cm slices.