Cut the chicken into thin slices. Mix the coriander, pepper, lime juice, sugar, garlic and oil. Toss the chicken in this mixture, then thread on to 12 bamboo skewers. You can make these up to a day ahead and chill until ready to cook.
Make the dip. Heat the vinegar and sugar in a small pan until the sugar has dissolved, then increase the heat and boil for 3 minutes, until slightly syrupy. Remove from the heat and stir in the chilli and shallot. Leave to cool.
Thinly slice a 5cm piece of cucumber, quarter each slice and add to the dip. Cut the rest of the cucumber into thin sticks.
Cook the chicken under a preheated grill for 3-4 minutes each side, then serve with the dipping sauce, cucumber sticks, rice and pak choi.