A contractor died yesterday after he fell into an eastern Henrico County sludge vat that was being re-roofed, officials said. Henrico police were called to the county wastewater plant in the 9100 block of WRVA Road at 8:47 a.m. for a medical emergency, said Lt. D.D. Engels. Engels said a man who was working at the plant had fallen into a vat used to hold semisolid sludge.