How should I modify my vanilla frosting to convert it into a chocolate frosting?
Whatever your type frosting is, ie Italian meringue or confectioners' frosting, you can simply add enough (cooled) melted chocolate or cocoa powder (sifted to remove lumps) until it tastes good to you. Since neither cocoa powder nor melted chocolate will curdle the frosting it is simply a matter of taste. However, if you are going to refrigerate the frosting, remember that the chocolate will solidify and thus your frosting will become firmer.

For some rough guidelines I recommend  1/4 cup cocoa powder for every 2 cups of frosting and 3 ounces melted chocolate for every cup of frosting.

Use a good quality dark chocolate, with a high proportion of cocoa solids, for a strong chocolatey taste. Or, if you don't like dark chocolate, use a half and half mixture of milk and dark chocolate.