DIY E-Liquid
Great resource and perfect place to ask this question and get loads of input
In short, it depends. Is it real or imitation? In the ingredients, is there anything but vanilla, water, and alcohol? If it is just vanilla, water and alcohol, then yes it can be used. I'm not sure of the results. Tasting vanilla extract, all I get is an alcohol taste
Check out the link above, there are many other options for vanilla flavoring that, imho, would give a much more satisfying result.