Zollinger Ellison and Prevacid

My 80 yr old father was diagnosed with Zollinger-Ellison syndrome about 15 years ago.  After an operation on his stomach (and an attempt to remove some of the gastrinoma tumors), he has been on high does of Zantac/ranitidene.  His is now taking about 675 mg of ranitidene (in 3 doses a day) which seems to control his acid level ok.  My questions are:

Will this level of acid supression lead to problems in absorbing any nutrients?  He has had a bout of shingles (with residual PHN) and I wonder if he is absorbing enough vitamin B12.  [Is there any form of vitamin B12 that would be absorbed better?].  

Also, I know that prevacid (and prilosec) appear to be used to treat Z-E these days, and I wondered if anyone has had any experience in switching and if the new drugs are as effective.

Thanks
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