Good advice from everyone so far! Take your own vitamins- they can get an order from the doc stating you may use your own home meds. They may keep the meds and dispence them to you, or may let you keep them; they may also want to send the bottles to the pharmacy in the hospital to verify that they are indeed what the label on the bottle says, then return them to you. (People try to sneak in their own narcotics and street drugs, so they have to verify the medications.)
As for meals, ask for 6 small meals instead of the 3 large meals. You should also talk with the dietician- they will be able to help you make choices from the generic menu. And of course, take what you can with you. The foods you know you can tolerate will be comforting to you.
Nurses are becoming more and more familiar with the special needs of WLS patients, so don't let it worry you too much. You will be on a surgical floor and chances are they have taken care of post WLS patients before.
Good luck! And don't sweat it!