How to Make It
Light a grill. In a small saucepan, combine the vinegar with the sugar, garlic and chiles and bring to a boil. Transfer to a small bowl and let cool. Stir in the fish sauce and 1 tablespoon of oil and season with salt and pepper.
Rub the lamb all over with oil and season with salt and pepper. Grill over a medium-hot fire, turning, for 25 to 30 minutes, or until nicely charred and an instant-read thermometer inserted into the thickest part registers 135° for medium-rare meat. Transfer the lamb to a carving board to rest for 10 minutes.
In a large bowl, toss the carrots with the cucumbers, jicama, bean sprouts, mint, cilantro and lemongrass. Add the dressing and toss to coat. Transfer the salad to a platter. Thinly slice the lamb. Arrange overlapping slices on the salad and serve.