For making vinegar tinctures, the folk method is generally preferred but you can also do the weight/volume ratio method.
In order for your vinegar tincture to be preserved, it needs to be at least 5% acetic acid. Apple cider vinegar on its own contains 5-6% acetic acid.
Therefore, you don’t want to introduce any more water into your menstruum, as this would dilute that percentage. For this reason, it’s recommended to only make vinegar tinctures with dried plant material, as fresh material has lots of water inside it that will enter into the menstruum. This also means that you don’t need to add any water to the vinegar.
It’s also recommended that you warm your vinegar and plant material before straining it, which can increase its shelf life.
You can try a vinegar tincture with fresh plant material that has been wilted if you plan to use the tincture quickly and store it in the fridge.
You may also like to explore glycerine as a menstruum! You can actually mix glycerine into vinegar to increase its shelf life, as glycerine is a preservative and a menstruum for extracting herbal constituents.
A book that’s helped me a lot with understanding herbal extraction is The Herbal Medicine Maker’s Handbook, by James Green. I highly recommend getting a copy if you don’t have one yet!