You will do just fine! I recently got married, and we wrote our own vows. I got through them. :) I don't know anything about Catholic Weddings, but I assume you traditionally repeat the vows that the priest will say? You might not even stutter repeating! I personally often time have an easy time with that, but of course you know your stutter. But no one will laugh. These people love you and are there to support you. <3