Tincture is chemistry. It's the separation (leaching) of the medicinal properties from the plant. What you use will determine it's strength. The higher the proof of the alcohol used, the higher the potency of the tincture.
The only way to achieve the higher quality of potency is by using Vodka to as close to 100 proof as you can get. It's the reaction of the plant to the contents of the Vodka that gives you the best product.
Vodka simply contains the best leaching abilities. Any alcohol will work, to some extent, but your end product will be different. It isn't that you don't end up with 'medicine', rather you will end up with a less potent form of medicine.
There's one caveat, however. You would be better off using 90 proof rum than you would 50 proof Vodka. Although Vodka 'draws' the medicinal qualities better, the amount of alcohol content is just as equally important. But either way, you won't end up with best quality of product unless you use Vodka which is at least 80% (again, the higher proof the better).
Now lower potency's will still work for some people; but not 'most' people. Most people will react much quickly and, with less dosages, and the more effective the tincture will be if it is more potent.
Think of it this way. Someone will do fine with a traditional prescription medication that is 100 mg; where it won't do a thing to someone else and they end up having to take 500 mg. It just depends on the nature of your illness, how far advanced it is, what else it is effecting, and just the personal make up of each of our bodies.
Or in a totally different way, think of it in terms of laundry detergent. For some, they don't need a high potency detergent to get their clothes clean. They're clothes aren't stained or as badly soiled. For another, however, they need not only laundry detergent, but they need more of it and with the kind that also contains bleach. Their clothes are just too badly soiled and the lower amount of detergent just won't work for them. It isn't that either type of detergent doesn't work, it's just they don't work equally well for both people.
That's the problem that discourages the use of alternative medicines/treatments. We want instant health and some people (including physicans) don't want to take the time that is often required of finding just what works for 'them'. There is no universal dosage, nor even universal herb for the same condition for everyone. It's trial and error and it's often time consuming and difficult when your already feeling really bad. But once you hit on what works for 'you', it's nothing short of a miracle from there on out because it is sooooo effective!
my experiences differ from yours. Which 
