Throughout my two post-op pregnancies I never took any officially "prenatal" vitamins. I just stuck with my usual regimen which already included the ever-vital folic acid supplement. I was already on the folic acid because I was told (somewhere/by somebody) that taking it can help with getting pregnant.
I had trouble with my second pregnancy (twins) keeping food, let alone vitamins down. I finally found a multivitamin that I would not throw up. I have all sorts of information about this on my website:
http://www.MyGastricBypassLife.com/vitamins.
I would suggest getting on folic acid ASAP and also looking into a DHA supplement. This is an Omega 3 precursor which is said to be critical to brain development. There are also some prescription vitamins that you can ask your OB about.
My advice I give to all post-op pregnant mothers: talk with your OB early about alternatives to the "sugar test" for gestational diabetes.