Preheat a grill to medium high. Microwave the sweet potatoes until just starting to soften, 8 to 10 minutes. Let cool, then slice into 1/2-inch-thick rounds.
Whisk 2 tablespoons vegetable oil, the cumin, chili powder, 1 teaspoon salt and a few grinds of pepper in a large bowl. Add the potato slices and gently toss to coat. Transfer to the grill and cook until charred, 2 to 3 minutes per side. Transfer to a platter.
Mix the cilantro, remaining 1/4 cup vegetable oil, lime zest and juice and chipotle in a bowl; season with salt. Thin with more lime juice if needed. Spoon the dressing over the sweet potatoes.