To prepare shrimp, combine first 8 ingredients in a zip-top plastic bag. Add shrimp, and seal the bag. Refrigerate 2 hours, turning the bag occasionally. Remove shrimp from bag; discard marinade. Pat shrimp dry.
Heat a grill pan coated with cooking spray over medium-high heat. Add half of shrimp to pan, and cook 2 minutes on each side or until done. Repeat procedure with remaining shrimp.
To prepare salad, arrange the lettuce leaves on a serving platter. Top with tomatoes and shrimp.
To prepare vinaigrette, combine 1/3 cup buttermilk and the remaining ingredients, stirring with a whisk. Drizzle vinaigrette over the salad.