Vitamins — how did they get their names?
They were more or less named in the order that they were found.  Letters F-J are skipped because many of those were reclassified or later found not to be vitamins.  The K comes from the German word for Koagulation because the vitamin is involved in the coagulation of blood.

The discovery dates of the vitamins and their sources /
Year of discovery   /Vitamin    Food source


1913 Vitamin A (Retinol) Cod liver oil
1910 Vitamin B1 (Thiamine)   Rice bran
1920 Vitamin C (Ascorbic acid)   Citrus, most fresh foods
1920 Vitamin D (Calciferol)  Cod liver oil
1920 Vitamin B2 (Riboflavin) Meat, dairy products, eggs
1922 Vitamin E (Tocopherol)  Wheat germ oil, unrefined vegetable oils
1929 Vitamin K1 (Phylloquinone)  Leaf vegetables
1931 Vitamin B5 (Pantothenic acid)   Meat, whole grains, in many foods
1931 Vitamin B7 (Biotin) Meat, dairy products, eggs
1934 Vitamin B6 (Pyridoxine) Meat, dairy products
1936 Vitamin B3 (Niacin) Meat, grains
1941 Vitamin B9 (Folic acid) Leaf vegetables
1948[42] Vitamin B12 (Cobalamins)    Liver, eggs, animal products


There are many different B vitamins, water soluble, because many vitamins previously given different letters where later found to be part of the B complex. For example H is now B7.  Additionally, many letters, such as P, are no longer classified as vitamins.

More information can be found on this Wikipedia page.