What does "adequate hydration" have to do with preventing vomiting? [not dehydrat from, paul cohen? ]

Nothing. When vomiting you must provide adequate hydration; otherwise you will become dehydrated. If you are not able to keep fluids down and replace the losses you would need to be in the hospital getting IV fluids until the problem is passed and you can again take your fluids orally. To prevent vomiting you may need assistance with medications such as phenergan (promethazine).