A tumor that secretes vasoactive intestinal peptide, a neuropeptide that causes vasodilation; relaxation of smooth muscles; watery diarrhea; hypokalemia; and HYPOCHLORHYDRIA. Vipomas, derived from the pancreatic ISLET cells, generally are malignant and can secrete other hormones. In most cases, Vipomas are located in the pancreas but can be found in extrapancreatic sites.