Combine black pepper, salt, paprika, garlic powder, onion powder, rosemary, and cayenne in a small bowl. Rub on to all surfaces of the tri-tip. Cover and refrigerate for 3 hours. Remove and let sit out for 30 minutes at room temperature. Combine mustard, vinegar, vegetable oil and crushed garlic in a jar with a lid and shake vigorously until emulsified. Prepare charcoal or gas grill.
Place meat on grill and brush with oil and vinegar mixture. Turn every 3 to 4 minutes, basting generously each time. Grill 25 to 30 minutes for medium-rare (internal temp of 135F.). The outside of the roast will get very dark and develop a charred crust—this is desired and one of the signature characteristics of this barbecue recipe. Remove meat from grill to a large platter. Let rest 15 minutes before slicing in 1/4-inch thick pieces across the grain. Serve as you would any barbecued beef, but in California this is often served with tortillas and salsa.