Pour the vinegar and lemon juice into a small bowl. Then add in the mustard, and whisk until well combined. Add in the garlic, oregano, basil, salt, pepper, and sugar, and whisk again until all combined. Whilst whisking, slowly add the oil, ensuring it emulsifies into consistent dressing.
The chicken should be jointed into drumsticks, thighs, and the breast (on the bone) removed from the lower carcass and cut into two pieces, giving 8 pieces in total. Place in the ziplock bag, add in the dressing and massage it around the chicken to ensure it's well covered. Chill in the fridge for at least 4 hours, ideally overnight.
When you're ready to cook, remove the chicken from the marinade, shaking off any excess but ensuring each piece is coated. Cook over a medium heat grill, be it BBQ, firepit, or grill pan, until golden brown all over and cooked through (internal temp of 75C, and any juices run clear).