I disagree. Does water have to reach boiling temperature before any water evaporates? The boiling point is simply the temperature where the stuff's vapor pressure equals the atmospheric pressure. It's not like there is zero vapor pressure below the boiling point.
Cast in a well ventilated area and try to sit upwind of the casting pot.
vapor is a bad terminology for lead. When lead is "fuming" it can be absorbed through the skin Lead is a toxic material with a low melting point (621˚F) and boiling point (3164˚F). Lead fume generation starts when lead is heated to about 900˚F. The fume generation increases as the temperature increases. Casting is done usually between 650-700° F.
This makes casting relatively safe.