heartbeat... you seem to have the theory down for casting shadows from an artificial light in perspective. the way to figure out where your shadow vanishing point is is drop your artificial light source vertically to the floor. it's that easy. you drop it where ever you want your light source to be in space. longer drop the the floor and your light source is closer to the viewer.
i whipped up a quick basic example:
for sunlight when the sun is in FRONT of the viewer, you just drop the position of the sun onto the horizon line and that gives your vanishing point, for sunlight when the sun is BEHIND the viewer drop the light source (sun) to it's inverse position below the horizon line (for exmaple, sunlight behind and left of the viewer has the light source placed below the horizon on the right side of the view) then pull your shadow vanishing point UP to the horizon line and intersect ray lines and shadow lines like usual.
now, keep in mind that this is a real quickie thing and there are more rules for shadows cast on vertical surfaces, over round or complex shapes, etc. that link that idiot apathy put up is a really good resource once you understand the idea and can wade through the terminology.
***edit*** for the record, you shouldn't really have the VPs on the page, as the cone of vision is really only a small area between the VPs, they're just here for demonstration