Leave whatever you didn’t use in the slow cooker (juices and onions).
Remove all the bones from the entire chicken and place in the slow cooker with any additional chicken parts you have, including the neck from the chicken you just cooked. Add vegetables, vinegar, and enough water to come up to about an inch from the top.
Cover and cook on LOW for 8 hours or overnight. My slow cooker automatically goes to WARM mode after 8 hours.
Strain and use immediately if you don’t see too much fat at the top or cool slightly and refrigerate. When cold, skim and discard any fat from the top. Stock can be kept in the refrigerator for up to 4 days or frozen for up to 3 months.