For the vinaigrette:
Blend the vinegar, water, onion, cilantro, honey, salt and pepper, to taste, in a blender until smooth. With the motor running, slowly add the oil and blend until emulsified.
For the potatoes:
Whisk together the chili powder, cinnamon, cumin, sugar, salt and pepper, to taste, in a small bowl.
Heat the grill over medium-high heat.
Brush the potato slices with oil on both sides and season with the spice rub on one side. Skewer 2 slices of the potatoes onto 2 skewers (so the potatoes lay flat) and place on the grill, rub side down and cook until golden brown and a crust has formed, 2 to 3 minutes. Turn the skewers over and continue grilling until the second side is golden brown and the potatoes are just cooked through, 2 to 3 minutes longer. Remove to a platter and immediately drizzle with green onion vinaigrette, green onions and chopped cilantro.