Vanadyl sulfate is a commerical derivative of vandium which is a trace mineral found in vegetables and fish. The body doesn't need much vandium and at high doses it can be VERY toxic and may cause excessive fatigue.
Theory: Vanadyl sulphate is suppose to have a tissue building effect by facilitating motion of glucose and amino acids to the muscles faster and raising insulin to promote growth. Most of the positive studies have only been shown in rats.
Does it work? In one 12 week study with 40 participants, they found that there was not a significant correlation to indicate that vanadyl sulfate did increase lean body mass. However, there were some modest improvements in strength training within the first month. Twenty percent of these participants experienced serious fatigue during and after training while on the supplement.