A flash/separation vessel for treating hydrocarbon feedstock containing resid to provide a liquid phase and a vapor phase has: (A) an inlet for introducing hydrocarbon feedstock; (B) an inlet for adding heated vaporous diluent to the vessel to dilute the vapor phase; (C) a flash/separation vessel overhead outlet for removing the vapor phase as overhead; and (D) a flash/separation vessel liquid outlet for removing liquid phase as bottoms from the vessel.