When I make my Lobelia tincture I tincture half in vodka and half in ACV, after they've tinctured for a long time I mix the two together.
I tinctured Solomons Seal in just vodka and wow, that is a powerful herb! Its also very cool to watch it when you shake it due to it being resinous. I've never heard that you couldn't/shouldn't tincture it in just vodka, I'd be interested to know where you learned that. I've tinctured many gallons of that because its one my husband takes daily, he doesn't want me to ever run out of it lol.