Clarification requested. Is 1/3 c oil to 1 TBS vinegar correct way of reading instructions?

Best to think of it as “parts,” Any measuring device will work. TBS, teaspoon, 1 cup, 1/2 cup, 1/4 cup, old Trader Joe’s mustard jar, etc. depending on the desired amount of finished dressing. For a small amount of dressing use a tablespoon. I like to mix up a large amount at once so I often use a 1 cup measure. A very common dressing might be 1 part vinegar to 3 parts oil. This ratio can then be adjusted to taste - a bit more acid/vinegar, maybe a bit more oil. The recipe suggests 1 Tablespoon vinegar and 3 Tablespoons oil to make about 1/3 cup finished dressing. Best Regards!