I'm very unimpressed with the Vanilla flavorings I have, including INW Shisha Vanilla. I like to cook and bake, and I use real vanilla beans pretty frequently. They're expensive, but... real vanilla has a wonderful complex flavor that straight vanillin just can't reproduce.
The only Vanilla I've tasted in juice that I thought really good was in a Virgin Vapors bottle. They use Medicine Flower concentrates. I have quite a few MF flavors from ecigexpress, and they are head and shoulders above all other flavors, IMHO, but difficult to work with. Unfortunately, I was late enough to the party that ecigexpress was out of the MF Vanilla when I ordered. And, unfortunately, MF sells it in underweight 15 mill bottles, or $22.00 each plus shipping... that said, if you want a real vanilla and are willing to pay for it I think they might be the best option. I'm about to break down and spend the money to get a couple of the MF flavors I am missing, and Vanilla is at the top of my list.
Failing that, I have heard good things about the DIYFS Holy Vanilla. Unfortunately it is also a bit expensive to order, as you have to get it from them these days (at least in the States) and their shipping is not cheap for a single bottle...