Mix vinegar and sugar in a saucepan and bring to a boil until sugar dissolves. Leave the vinegar syrup to cool completely.
Peel the lotus root then slice thinly. Rinse the sliced pieces and drain dry in a plastic colander. Place the lotus root slices in a porcelain bowl. Add cili padi and pour in the cooled vinegar syrup. Add in lemon juice.
Cover the bowl with cling film and leave in the refrigerator overnight. Dish out and add a sprinkling of sesame seeds before serving.