I have a history of stomach ulcers. No recent problems. Recently after only about 5 units of alcohol I am very sick with stomach pains.

That's a message! If you pour alcohol over a cut on your skin, it burns, right? No surprise that if your stomach is inflamed, then pouring alcohol into it will make it burn too. Vomiting is one mechanism your stomach tries to rid itself of a toxic substance--in this case your vodka. It is a wonderful time to curtail consumption. Please do yourself the favor ; stop.