Ibn al-Qayyim said in "the Prophetic Medicine" regarding vinegar:
Muslim relates in his Saheeh, from Jaabir bin Abdullaah that the Messenger of Allaah (sallallaahu alaihi wasallam) asked his family for a condiment and they replied, "We do not have anything but vinegar". So he called for it and began to eat, saying, "How good a condiment is vinegar". In Sunan Ibn Maajah, from Umm Sa'd (radiallaahu anhaa), from the Prophet (sallallaahu alaihi wasallam), "What an excellent condiment is vinegar. O Allaah bless vinegar, for it used to be the condiment of the Prophets before me. No house will be destitute that has vinegar."
Vinegar is a combination of heat and cold, of which the latter is predominant. It is dry in the third degree, and has a strong dessicative power. It prevents matters from flowing down and refines the constitution.
Wine vinegar is beneficial for inflamed stomach, calms the yellow bile, and repels the harm of deadly drugs. It dissolves milk and blood when these have solidified in the interior. It is beneficial for the spleen, tans the stomach, restricts the belly, quenches thirst, and prevents incipient inflammation. It helps the digestion, opposes phlegm, refines thick foods and thins the blood. When drunk with salt, it is beneficial for one who has eaten deadly fungus.
When made into a broth, it dislodges the leech which has attached itself to the root of the palate. When warmed and used as a mouth rinse, it is beneficial for toothache and strengthens the gums.
It is beneficial for whitlow (pus-filled blisters) when applied as an embrocation, and for itching, hot inflammation, and burns. It increases the appetite, sweetens the stomach, and is good for young people, and in the summer for the inhabitants of warm lands.
Source: Zaad al-Ma'aad (4/280) and "The Prophetic Medicine" (P. Johnstone, p. 220).