1. In a large nonstick skillet or a wok, heat 1 tablespoon of the vegetable oil over medium-high heat. Once the oil is shimmering, add the crisped rice cereal and cayenne and stir until browned just slightly, about 2 minutes, turning down the heat if necessary to prevent burning. Transfer to a bowl and wipe out the pan.
2. In a small bowl, stir together the fish sauce and sugar.
3. In the same skillet, heat the remaining tablespoon of vegetable oil over medium- high heat. Add the pork and cook, stirring frequently to break the meat into small pieces, until it is no longer pink, about 5 minutes. Add the garlic, chiles and shallot, stirring to evenly distribute them with the pork, and continue to cook, pressing down firmly to help the pork brown and stirring just once or twice, until the pork is golden and crispy, 3 to 5 minutes. Drain and discard any excess oil. Stir in the fish sauce–sugar mixture and cook a few seconds longer to warm through. Off the heat, add 2 tablespoons of the lime juice. Taste and adjust the heat, acidity and sweetness if needed. Let cool for about a minute, then toss in the scallions, cilantro and mint.
4. On a large serving platter or individual plates, arrange a layer of lettuce leaves, then top with some of the pork mixture and cucumbers. Continue layering in the same way until you’ve used all of your ingredients. Scatter the crisped rice over the top. Serve immediately.