The answer is a bit more subtle. The word variety is a noun, meaning (in this context) "A plant or animal differing from those of the species to which it belongs in some minor but permanent aspect." The word varietal is (originally) an adjective, meaning "pertaining to a distinct variety of animal, plant, or mineral." This adjectival meaning came specifically to refer to wine: "Of wine: made predominantly from a single variety of grape." This word was used in the phrase "varietal wine."
However, take a look at this list of words: Dirigible, terminal, aerial, thermal, final, social, special, principal. These all may function as nouns. Clearly, we used to say:
The words in my list were all adjectives that modified the following noun. Eventually, we dropped the noun, and used the adjective as a noun. I would argue that varietal is in the middle of this process. The earliest entry in the Oxford English Dictionary for the use of varietal as a noun is from 1955, with the meaning "A wine made from a single variety of grape."
There are words that have been used this way for so long we no longer can even see it, like capital. There are others that are still early in the process: prenuptial is practically a stand-alone noun by now.
So, back to the original question: use varietal (as a noun or adj.) when referring to a wine or its characteristics. When referring to a grape, use variety, or, increaslingly commonly, varietal as a noun.