I have thought this exact thought for probably most of my life. In some ways, I'm dreading the idea of my wedding day for this reason, however ridiculous that may sound. I think the best way to ensure you don't stutter is to practice! Now you might be thinking "wtf does he mean practice". Well its like any other presentation you were forced to do in school. As a stutterer, I would memorize the SHIT out of my presentation. I would practice is 50-60 times until every single enunciation and tone that came out of my mouth was fluent. Not sure if this is the same for everyone who stutters, but for me, reading off a page rarely makes me stutter & practicing what I'm going to say makes me not stutter. It's been like this my entire life. Practice your vows and it will be second nature once you're up there.