Hey there. What I did that worked for me was to add 6 drops of vanilla extract to about 5ml of E-Liquid regular tobacco taste. It was just a guess, but I liked it. I had half a bottle of E-Liquid with the blah regular taste and kicked it up a notch. I haven't done further tests, but intend to. What I really want to do is smoke the Bickford flavors straight -- no nicotine liquid at all.
Just beware the ingredients of the extract you use. No oils. That's the big one. You are coating your lungs with whatever you inhale, so stick with PG or alcohol-based extracts. Bickford is PG based. Commerical E-Liquid is PG-based.
I haven't tried to add anything to my 30ml vial. I think what I'd do is squirt about 5ml in a spare bottle, then add vanilla. Try some with the 6 drops. Maybe you want stronger. Add more drops. I remain underwhelmed by every commerical "taste" I've tried. Tonight I squeezed liquid from "fruit" flavor cartridges -- orange, pineapple and melon into a new mix. It's tasty but so weak I can't really say I'm impressed.