Check whether you need (1) strong bean flavor, (2) speckles, and (3) paste-like thickness; pick the substitute that matches the most important of those.
Use the listed ratio, then stir into wet ingredients first so the vanilla disperses evenly (especially for powder and seeds).
If the swap is thinner than paste (like extract or syrup), reduce another liquid in your recipe slightly or thicken with a small amount of cornstarch for custards/creams.
If the swap is too sweet (vanilla sugar or syrup), reduce added sugar to keep the final dessert balanced.
Taste the batter/cream before baking; adjust with a few drops of vanilla (or a pinch more powder/seeds) rather than doubling all at once.