Lightly spray the inside of the slow cooker with cooking spray,
In a microwave-safe bowl combine onion, minced garlic, vegetable oil, 1 teaspoon of salt, and red pepper flakes. Microwave, stirring occasionally, until the onion is soft, around 5 minutes. Transfer to slow cooker.
Add collard greens and chicken broth and stir until blended. Place ham hock in the middle of the greens.
Cover and cook until the collard greens are tender, either on low for 9 to 10 hours or on high for 6 to 7 hours.
Once collard greens are cooked, transfer ham hock to a cutting board. Allow to cool somewhat and use two forks to shred it into bite-size pieces. Remove any fat, skin, and bones.
Stir the shredded ham and cider vinegar into the collard greens and season with salt, pepper, and additional cider vinegar to taste.