Vinegar can be used to treat hibiscus, but it is not recommended. While vinegar can be effective at killing some types of pests and fungi, it can also damage the leaves and roots of hibiscus plants. Additionally, vinegar is highly acidic and can alter the pH of the soil, which can be harmful to the plant. It is best to use a safer and more effective treatment method, such as a homemade insecticidal soap or a commercial fungicide.