I haven't used TFA but I have vanillin crystals that I've dissolved into a 10/90 ratio, and it's more than enough for a good mixer.
I'm at 10% Vanillin to 90% PG, and use typically about 7.5% depending on the recipe; and it gives it a good undertone imo.
I think for something where you want vanilla to be a BIG presence in the flavor, you'd have to go towards about 25% or more, as vanilla is really a subtle flavor, and can be overpowered by most anything.