If you go to the TTB website (http://www.ttb.gov/spirits/bam/chapter4.pdf ) you will see a chart describing all the Spirit categories. At the top of this chart is the Class category for NGS (neutral grain spirit) and a definition:
Spirits distilled from any material at or above 95% alcohol by volume (190 proof), and if bottled, bottled at not less than 40% alcohol by volume (80 proof)
Next comes the Type of Spirit, Vodka, and a definition:
Neutral spirits distilled or treated after distillation with charcoal or other materials so as to be without distinctive character, aroma, taste or color
Although the definition states, “…without distinctive character, aroma, taste or color”, I believe that to be, mostly, an erroneous statement. I will grant you the no color. Vodka is a clear Spirit (although I did see a barrel aged Vodka the other day). As for taste, character and aroma; Vodkas have always had their specific tastes, characters and aromas. If you pay attention you can smell and taste the difference in Vodkas made from various ingredients (i.e.: Wheat, Rye, Corn, Grape, Potato, Rice or even plain sugar).
I, personally, classify being smooth as a character trait. If all the Vodkas in the world met the TTB’s criteria they would all taste and drink exactly the same. Yet they don’t.