If I wanted to fix concrete vase how would I mix the concrete
I would probably try to use a super glue to rejoin the pieces. However, it you can't do that (missing pieces), I would try mortar mix.
Most repair concrete add a little water and the bag or container will tell you how much. There is also one that comes in a tube so that you can use the caulking gun.
Ken is right. Use superglue, epoxy, or Gorilla Glue. Check at your hardware store . They should have small tubes.
Mix with water in a small container see how thick you need so it will stay on side