To make the marinade, whisk all the ingredients together and season. Thoroughly coat the chicken in the marinade and leave for 6 hrs, or preferably overnight.
Remove the chicken from the fridge 30 mins before cooking. Heat oven to 180C/160C fan/gas 4. Tip the chicken and its marinade into a roasting tin along with the vermouth. Cover the tin with foil and roast in the oven for 1 hr 40 mins.
Meanwhile, heat the remaining oil over a medium to high heat in a frying pan. Put the chicory and orange halves cut-side down in the pan, and cook for 5 mins until caramelised. Set aside until the chicken is ready.
Remove the foil from the roasting tin and add the caramelised chicory and oranges, squeezing over the juice of one. Turn the heat up to 200C/180C fan/gas 6 and cook for a further 15 mins, until the chicken is nicely caramelised. Remove the tin from the oven and transfer the chicken, chicory and oranges to a plate and keep warm.
Put the roasting tin over a medium heat on the hob, and squeeze in the juice of half a caramelised orange, adding a little water and using a wooden spoon to scrape up any crispy bits. Cook until the juices are reduced to a sauce that coats the back of a spoon. Toss the cooked spelt through the juices, then add the parsley. Taste and season, adding the red wine vinegar, if you like. Divide the spelt between two plates and top with the chicken, chicory and half a caramelised orange to squeeze over.