Did you know that you do NOT have to exchange vows? Nope. Not required in any state that considers marriage a legal agreement. And the last time I checked, the officiant is only required to hear and acknowledge from you that you are both desiring to enter into that agreement of your "own free will". And that same officiant must hear you say "YES" or "I do" in some fashion in order for him/her to be able to legally sign and validate your marriage.
So what does that mean when it comes to writing and exchanging your own vows then? Yes..it is traditional to exchange vows. That's what folks are putting on their fancy clothes and coming to see you say. (Besides the custom-made cocktails of course.) It's traditional. It's what's expected.
But can't you just say "I do" and be done with it? Yes, of course you can. A skilled officiant and wordsmith will craft unique vows for you, put a spin on the "traditional" ones we've all heard hundreds of times or give you guidance as to how to write your own. Here's my two cents worth:
Keep it "clean" and relatively short. If you want to have the officiant ask you questions and you simply respond "I do", then usually 3-6 questions will suffice.
If you write a paragraph to read yourself, keep it to about 150-200 words. No more than the front and back of a 3x5 index card.
Some couples do shorter, and some do longer, but this is the normal range that I see from my couples. I recommend no longer than 300 words, as that feels really long to the guests listening to you read them. You also might be more emotional than you think, and therefore that is a long time to read while emotional and laughing or tearing up.
When couples have so much to say and have written something much longer, I often suggest printing out the entire version as a love letter to give their spouse either earlier that day and then use an edited-down version for in front of the guests.