SWEET POTATO, DATE & AUBERGINE TAGINE

2 tbsp olive oil | 2 aubergines, about 500g, cut into 2cm dice | 3 red onions, chopped | 1 tbsp vegan or vegetable bouillon powder | 700g sweet potatoes, cut into 2cm dice | 75g dates, roughly chopped | 1 tbsp ras el hanout spice | 400g can Essential Chickpeas, drained | 1/2 x 25g pack coriander, chopped | 2 tsp white wine vinegar