I am told it is potatoes. But that may be a rumor. Nope, not a rumor, I just went and looked it up. The finest vodkas are made from russet potatoes. Some vodkas can be made from rye or corn, but the distillation process is different and harder to get the purity of potato vodka.
"Vodka may be distilled from any starch/sugar-rich plant matter; most vodka today is produced from grains such as sorghum, corn, rye or wheat. Among grain vodkas, rye and wheat vodkas are generally considered superior. Some vodkas are made from potatoes, molasses, soybeans, grapes, rice, sugar beets and sometimes even byproducts of oil refining or wood pulp processing."
Yes Vodka is from fermented potatoes. I believe there are requirements for Vodka labeling in some areas, such as that it has to be just about nearly pure ethanol and water.
For example, the 180 proof Everclear was basically just ethanol and water. It was popular for cleaning lab glassware and other purposes. But then recreational use at that large college resulted in it being hard to obtain. Now if the person had drank a quart of xylene or toluene I wonder if we would not be able to buy those at the hardware store either.