Don't fret, I wrote mine sitting in a hammock in the jungle a few hours before the wedding and they were perfect. There are no rules that say they need to be done in advance, and if there were I would tell you to break them anyway
Write them when you are in a spiritually open place, whenever that may be. You are about to spend the rest of your life with the person you love the most, think about that and the words will come. If you can't get them right in advance I guarantee the words will flow on your wedding day.
My vows were written to my hubby with a few inside jokes and didn't entirely make sense to the crowd, but they weren't for the guests, kwim? Have fun with it and know that there are no right or wrong things to say