nicotine is a known irritant to the stomach.
the way it enters your stomach is via your mouth and saliva.
because vapers need to take a long puff before inhaling, than analog users do, it allows more nicotine to be absorbed in your saliva, thus causing a higher dose delivery of the drug to your stomach, and thus having a greater sensitivity to hurt burns and later on stomach ulcers.
nicotine interferes your stomach protective mucus from forming, thus causes the mentioned problem.
you have 3 choices:
1-lower your nicotine in the juice.
2-shorten your puffs, via using an LR atty or triple coil cartomizer, or going to VG-all that will produce much more smoke in a shorter puff.
3-continue your omeprazole treatment, till your able to lower your nicotine consumption.
in either way, washing your mouth with water post vaping might help a bit.