Are you reciting vows that you wrote yourself? That's what my wife and I did, so I practiced, practiced, practiced what I had written. It was muscle memory by the time I was up there, and it went pretty OK. Also, I could sort of predict words more likely to trip me up and could write around them. Your loved ones there probably know you have a stutter, so it's not like they'll be super confused or whatever. They're there because they want to show their love and support. This is the best audience you'll ever have! And like every other time you've been nervous about stuttering in front of people, a year from now, no one will remember the words you said or how you said them, but they'll remember the day, the friends and family, and the joy of being there with you.