I have been making this for years with bittersweet chocolate and didn't know that I was being "healthy", I just prefer that kind of chocolate! I have never had a problem with the chocolate seizing after adding vanilla. Maybe try adding the vanilla to the peanut butter and then stirring it into the melted chocolate? Truly yummy stuff!
You do not have a good recipe to make puppy chow because of the ingredients that should not be included. Vanilla extract causes the melted chocolate to harden. I tried making it twice thinking maybe I did something wrong. It is in fact your recipe. It does not make puppy chow possible to make. You need to remove vanilla extract from your recipe.