Deciding to write your own wedding vows is a major undertaking and one that has to be carefully approached with an extraordinary amount of fore thought. In the final analysis, the determining factor is probably truly based on your level of bravery.
Conventional wisdom would dictate that you "stick to the script." Everyone associated understands that it's your day to honour one another and they're happy for you but they secretly want the bar to open and for food to appear on their plates. After all, they want something in return for the gift certificate to Home Depot that lurks with a hundred other gift certificates on a table in the reception area. You can blab all you want on the altar but if the food is cold you'll never hear the end of it.

In short, it can be difficult dealing with traditionalists. If you have a family full of them, you can avoid embarrassment by allowing the minister to conduct the ceremony and just repeat what he tells you. On the other hand, if you choose to recite your own vows be extremely cautious. All too often your assembly will view lengthy monologues as dreary "yada-yada" stuff and feel like they're eavesdropping. You don't want to impose on their patient, good nature by venting your soul. After all, you already have the job and it's a pretty good bet that you've previously told your betrothed that they "complete you."

Writing your own vows can be endearing and add immeasurably to your new beginning. Choose your words wisely, though. Cut to the chase. No one wants to hear about rainbows, sunrises or flying doves unless you want your guests to conclude that you were drunk when you wrote this blather. Keep it straightforward and simple. Avoid the clichés. Avoid the intimacy. Manufacture words that stir the heart and batter the brain. If you're going to announce to the world why you want to spend your life with this person, be practical. You don't have to say, "I'm marrying you for your money." That's a bit tacky. But you can say, "I can't imagine going through life without you." That'll keep them in their seats! If the food is cold, you might be forgiven.
As previously stated, it's a bravery call. If you have writer's block, there's always Shakespeare!