Pick the one truth
Spend 20-30 minutes hereBefore you write a word, pick the one true thing you most want your partner to hear on the day. Not your love story. Not a list of their best qualities. The quiet observation only you would make about them, the one you have been carrying for a while without saying out loud.
Most generic vows start by trying to summarize the whole relationship. The strongest vows do the opposite. They pick one specific small truth and let everything else live inside it. Examples of what counts: a habit your partner has that nobody else notices, a sentence they said to you once that you have remembered for years, the thing they do for you that they would never call a gift. Avoid the obvious picks. "How we met" and "everything I love about you" are setups for cliches because every set of vows tries to use them.
If you cannot think of the one truth, write down everything about your partner that does not photograph well. The undramatic everyday things are usually where the real material lives.