It's not optimal. Real juices are made with pro-strength flavorings. But as long as it is not artificial vanillin and does not contain any real sugars, you could use it. But it will be weak because if you add too much then your
juice will be too much alcohol and that won't
vape.
A slightly better choice might be the Frontier brand Fair Trade no-alcohol vanilla FLAVORING (not extract) found in many health food stores. Check the ingredients to make sure there is no corn syrup, no sugar (it is in VG though and will be quite sweet and thick).
A better choice is see if your Walmart has some Lorann Oils flavors (the little bottles) and avoid the ones that are actually oils (mainly nuts, some spices like cinnamon, and citrus flavors). See the DIY section of ECf guide for a list of Lorann flavors with the ones not good for
vaping or not water soluble marked.
http://www.e-cigarette-forum.com/forum/diy-e-liquid/