Determine how high you want to raise the vanity. For example, to raise the vanity 1 inch, a stack of shims equaling an inch tall will be required. Shims come in a variety of sizes and are sold in bundles at home improvement stores.
Lift the left rear corner of the vanity enough to wedge one or more shims underneath it. Allow the corner of the vanity to sit atop the shims, but move the shims back enough to hide them from view. Then repeat the process for the left front and right side corners.
Place a level on the back side of the vanity to make sure it is even. If one side is slightly higher than the other, insert a taller shim underneath the lower side until even.
Set the level on the left side of the vanity to make sure the left rear leg is even with the left front leg of the cabinet. If not, insert a taller or shorter shim in the front.
Place the level on the right side of the vanity to make sure it is even. Adjust the size of the front right shim if needed.