So I used vanilla to cut down some MLV DK Tab...
Split it in half, then filled both bottles back to level with Glycerine USP
Dripped about 2.5ML of the vanilla in each
First impression is the vanilla has a "baked" taste; Not the cleaner type of vanilla I was hoping for. Some may like it this way. Also, this is how it tastes out of the bottle, without vaping.
Another thing I'm not keen on is that the vanilla smell really lingers in the air, like nothing I've vaped before. This also lingered on the coil wick for some time, after being burnt off. My hands smelt this way too, even after washing with dish soap. My initial thought was that this vanilla flavouring contains sugar, but I know it doesn't after directly tasting it.
Anyhow, maybe I put too much in. Lessons learned to try a smaller sample

Oh well, I'm not huge on MLV Dk Tab anyhow, with the metallic aftertaste.
tl;dr: Vanilla is quite strong and has a baked taste.