A crack in a vase usually can be repaired with only a few supplies no matter what material a vase is made out of. This project does not require any expert knowledge or skills and can be completed within an hour, at the most. Sealing a crack in a vase can be done in a variety of ways with many different sealing materials.
Clean the vase, both inside and out, to be sure any dirt has been removed.
Use a silicone putty to patch thick cracks. Silicone putty is safe for use in bathrooms and areas with a lot of moisture. Silicone dries clear so no paint retouching will be needed. Silicone putty comes in a tube. Squirt the silicone putty into the crack, and then smooth the putty into the crack with a small putty or butter knife. Continue adding putty and smoothing until the crack is filled and smooth. Wipe off the excess silicone. Then, once dry, sand the area to remove any rough spots.
Apply a thin layer of clear nail polish to the crack on the outside of the vase. If you are able to reach inside the vase, apply a thin coat to the inner side, as well. If you do not have clear nail polish on hand, use superglue.
Allow the clear polish or superglue to dry for 10 minutes to 15 minutes.
Add some water to the vase to see if the crack is still leaking water. If it is still leaking, try adding a second coat of polish or super glue.
Display the repaired vase in a safe area. It will be more fragile now that it has been cracked.