Cook the onion and garlic in a bit of oil until the onion is soft.
Add remaining ingredients, except cider vinegar, to the pot.
Bring to a boil and cook until potatoes are tender, about 30 minutes.
Stir in cider vinegar, if desired.
To make soup in slow cooker, place all ingredients in crock and cook on high 5-6 hours or low 7-8 hours. No need to cook onion and garlic before adding to slow cooker.