When you vomit you are expelling gastric contents. The pH of the stomach is highly acidic because of HCL hydrochloric acid. HCL has a pH of about 0.8, remember that the pH scale is from 0-14. When you lose gastric contents, you also lose this very strong acid. Proton pumps in the gastric mucosa work to replace this lost acid but they can't replace it quick enough, especially if you vomit repeatedly. The pH of the stomach becomes more alkaline or basic(because you have lost the HCL). The microvilli of the mucosa is highly vascular(has a good blood supply). Now, the alkaline fluid or chyme is readily absorbed by the blood vessels in the stomach and transported to the bloodstream or cardiovascular system. Vomiting once or twice will not change the pH of the blood but intractable vomiting will.
Vomiting alters the acid-base balance by causing a massive loss of chloride from the stomach as hydrochloric acid (HCl). This causes an increase of bicarbonate in the extracellular fluid (any fluid in the body that is not part of the fluid in the cells). Information from: Thibodeau, Gary A. and Patton, Kevin T., Structure & Function of the Body, 2008, St. Lewis, Mosby Elsevier.