Toss the pork with half of the coconut milk, the vegetable oil, garlic, 1/3 cup each brown sugar and cilantro, and 1 tablespoon each fish sauce, lime juice and Sriracha in a large bowl. Cover and refrigerate 1 to 4 hours.
Preheat a grill to medium and lightly brush the grates with vegetable oil. Thread the pork onto eight 10-inch skewers. Grill the kebabs, turning occasionally and brushing with the remaining coconut milk, until marked and cooked through, about 12 minutes.
Meanwhile, combine 1 tablespoon water and the remaining 1 tablespoon each brown sugar, fish sauce, lime juice and Sriracha in a small bowl. Serve the kebabs in romaine leaves and drizzle with the sauce. Top with chopped peanuts, scallions, cucumber and cilantro.
If you're using wooden skewers, soak them in water for 20 minutes to prevent scorching.