Combine the vinegar, oil, soy sauce, mustard, minced garlic, salt, and pepper in a bowl.
Pat dry the steak, then put it in a shallow dish. Pour the marinade over the steak, turning to coat it evenly.
For best results, marinate for 15-30 minutes, then leave at room temperature. If you marinate too long, the acid may toughen the meat.
Prepare the grill by heating it and applying oil on the grill grates; this should be done at a medium-high level. Considering thickness, rest for 5 minutes if slicing against the grain, and grill for 3 to 5 minutes on each side.