I've never used Vanadyl Sulfate on its own, but it was a common ingredient found in many GDA products. Many GDA products have reformulated and removed this ingredient due to studies showing that it can have negative effects at higher doses including damage to DNA and blocking of protein synthesis.
As far as lowering blood sugar it is a very effective ingredient, but due to the research on the harmful effects I would suggest using a different GDA product. There are tons of great GDA/nutrient partitioners available that have a more complete profile and help to lower blood sugar and drive nutrients to the muscle. My favorites are Omega Sports Burn 24 and Performax Labs Slinmax - both of which contain Na-R-Ala which is my favorite ingredient for aiding in lowering blood sugar