Yellow leaves, blackened succulent stems and declining roots are different symptoms, so a single household remedy cannot explain every case. Check moisture, drainage and root condition before deciding what needs to change.
Begin with the growing conditions
A gardenia with yellow leaves and a succulent with a collapsing black stem do not necessarily share a cause. Before treating either plant, note the recent weather, watering and feeding, then check whether the growing medium is wet, dry or compacted.
Separate the observations
For gardenias, review the acidity of the growing medium as well as root moisture and light. Yellowing alone does not prove an iron shortage. For a succulent, inspect the base and roots for soft or damaged tissue. A dry leaf at the bottom of a rosette is a different observation from a spreading soft patch in the stem.
Avoid the universal remedy
Do not add vinegar or an unmeasured mixture of salts to every yellowing pot. Correct the condition that the inspection actually reveals, and use any labeled product only for its stated purpose. Keep the plant under observation, photograph changes and seek a more specific diagnosis if damage continues. New healthy growth is a better measure of recovery than the color of an old leaf.
Related reading: Peace Lily, Pelargonium and Coinwort: Separate Care Checks.