The flavors we use in vaping were actually designed for cooking, so yes...you can use cooking flavors. However, the vanilla extract we buy at the grocery store is usually not nearly as concentrated at the flavors we buy to use in our juices. Also, it may have quite a bit of alcohol in it....so it may really "water down" your juice. Certainly a little won't hurt anything....and may help make a bad juice better. Read the ingredients and you'll probably see a lot of alcohol. What you don't want to do is use a flavoring that has oil in it. For instance, pepperment oil is NOT to be used.
It's much better to go to a site that sells flavorings, and pick up something that might help several of your juices. For example, if you add Bavarian Cream to any fruity juice, it may help it.
E Cig : Best Ejuice | Joye eGo & eGo-T | Boge Leo | Cartomizers | E-Liquid | Electronic Cigarettes is a place I go to get many of my flavorings.