Vanilla extract is generally in alcohol, so would only be good for thinning a thick VG
juice. It would make PG too thin. Most people think a
juice made with vanilla extract is too weak in flavor at the thickness that is vapable. I have used Frontier Vanilla Flavor, which is vanilla extracts in glycerol, and is available from Whole Foods or online. Its ok, but rather weak. You can use it, however, in place of VG used to make a DIY
juice. Many here use Capella's Vanilla Custard flavoring for their vanilla DIYs. Its very good, but not really like vanilla ice cream vanilla, more like baked custard. Perfumer's apprentice has several vanillas, all of which are quite good, and at good prices.
Of course this is all assuming you are making juices DIY. I know of no extracts that are vapable on their own. And of course, none will have nicotine in them.