Isnt this an interesting cycle that ppis reduce stomach acid to help with gastritis but reduction of stomach acid decreases (for example) vitamin b-12 absorption that can also cause gastritis. what is way out?

All medications have the possibility of side effects. It is vital that before beginning meds, a patient try dietary changes (less fat, salt, spicy foods, fried foods), increased hydration (3 liters daily for a male) and stress reduction to control gastritis. If nothing else works and medications are necessary, B12 supplementation using an under the tongue, easily absorbed form, can help.

Acid control in the stomach per se does not interfere with B12 absorption, unless you have co-existing chronic gastritis. As advised by Dr. Kratzberg you can take sublingual B12 that bypasses intestinal absorption.