There is nothing bad about flavoring coffee with vanilla extract. Indeed, McCormick published a recipe for vanilla coffee that calls for about 1/3 teaspoon vanilla extract per cup of coffee.
McCormick's recipe calls for 2 teaspoons of vanilla extract and 1/4 cup of brown sugar in 6 cups of brewed coffee. Many coffee shops also routinely use vanilla extract in some of their recipes. Since vanilla concentration is a matter of personal preference, it is worth experimenting by slowly adding a drop of extract at a time to a fresh cup of coffee. Vanilla mixed with heavy cream has a flavor and texture similar to many commercial coffee creamers.