Thoroughly clean your vases removing all dust, lint, and price tag residue. You can use a sticker removal product such as this one, but in a pinch I find any kind of cooking oil or baby oil and dish soap works well too. Just be sure to wash off all traces of the oil.
Apply the adhesive gold leaf size according the directions. There's a curing time until the adhesive gets tacky, so wait it out until it's ready or the gold leaf won't adhere correctly.
Next, carefully (and I mean carefully -- those gold leaf pages are finicky little suckers) apply the gold leaf to your vase. Carefully smooth down with a soft cloth or clean paint brush. Cotton gloves also work if you already have them on hand. You can use any excess gold leaf that falls off to apply to the next spot. If you're not fussy and like an abstract or rustic look, don't worry about being too perfect.
Let your vases dry according to directions, or overnight.
Finally, apply sealer and let dry thoroughly before using.