Hello! Running out of vanilla extract mid-bake happens to all of us, but you have several fantastic substitutes available at any basic supermarket that will work wonderfully in a lighter cake.
If you want to keep a subtle, warm flavor profile, maple syrup or honey is an easy 1:1 substitute for vanilla extract. They provide a soft background sweetness and depth without changing the texture of your cake batter.
Another option is almond extract, which pairs exceptionally well with lighter cakes. However, keep in mind that almond extract is significantly more potent than vanilla. I recommend using half the amount of vanilla extract called for in the recipe so it does not overpower the cake.
If you happen to have fresh lemons or oranges, citrus zest is another bright alternative. Grating half a teaspoon of fresh lemon or orange zest into the batter will give your cake a clean, refreshing aroma that highlights light cake recipes well.
Finally, if you have dark rum, bourbon, or brandy in your pantry, you can replace vanilla extract 1:1 with any of these spirits. The alcohol bakes off during cooking, leaving behind warm vanilla and oak notes.
I hope your cake turns out great tonight! Please feel free to ask if you need help with specific measurements.