What Hulda wrote in her book included 2 examples: Vodka; Grain Alcohol. In her book, plain Vodka was the example given for making tincture from Green Hull of Black-Walnut s. In this context, plain means "not flavored". Vodka, plain as well as numerous kinds of flavored variations, is certainly found in bars. Depending on your local within this Union, Grain may be known as Grain, or Everclear.
The grain used for producing Grain Alcohol is commonly Corn, but many others can be used too.
Plain Vodka that is widely available for consumer purchase is likely to be anywhere between 80 proof and 100 proof. This kind of plain Vodka is ready to go to be used for making a tincture such as Green Hull of Black-Walnut . Plain Vodka is generally produced by using Grain Alcohol made into a a simple dilution. Grain Alcohol is generally 190 proof. Divide the proof by 2 to get the percentage of alcohol. 190 proof = 95 proof, but you might find some calling it 100 proof..... it's close enough for government work and historicially the government has had a strong hand in controlling the production, sale and taxation of alcohol. Historically, there was a time when the lions share of this kind of revenue was produced by guns, alcohol and tobacco....... but we're not in Kansas anymore Toto ;) The simple dilution often used is distilled water. By adding equal parts 190 proof and distilled water, the resultant "vodka" is 100 proof AKA 50% alcohol (technically, it's 95 proof, 50%). If a bit more water is added (instead of a 1:1 ratio), then a lower percentage alcohol is produced, perhaps something like 80 proof.
The same basic dilution formula is used for other types of commercial alcohol such as whiskey and bourbon. I think you'll find the same simple dilution is involved with many other flavors of alcohol, like Rum, Gin, etc., but these and other flavors also involve the use of flavoring methods, such as various sugars. Some of these flavors are then "aged" in wooden or charcoal casks to enhance the flavorings..... strange, I did not know nor care to know any of this back when I was a regular drinker. It's only been during some of the free time freed up since quitting that I've learned some of these facts :)
If you want to make your own tincture and have plain 80-100 proof Vodka, you are good to go. If you have grain, you should first dilute the Grain with equal parts distilled water, then you are good to go.