Open your Module 3 structure and your Module 4 promises side by side, and write straight through without stopping. Don't fix anything. Don't reread. Just get to the closing line.
On voice: read a sentence aloud, and if you'd never say it to your partner in a kitchen, cut it. Contractions are your friend. "I'm" not "I am." "You've" not "you have."
On length: 150–300 words. Roughly one to two minutes spoken. If you're over 350, you're not writing vows, you're writing a speech — and the good news is you can move the extra into your thank-you speech.
Read the draft out loud once, with a timer. Note where you stumbled. Stumbles almost always mark a sentence that isn't in your voice yet.