VIS will make only the areas* that are directly visible from the area* you are in get drawn. So if the area behind the skybox is not connected to the area you are in, it will seem like there's nothing there. For example on karith, looking up and to right out of the windows near elevator room, you should see alien default there, but VIS cuts it out (use noclip to see where it is). You can also /bind x toggle r_showtris to see which areas are being drawn at any time.
Tho generally you should avoid mapping like that or if possible map the outside walls of the areas visible thru skybox as well along with not blocking vis to those outside walls (but possibly having those walls be structural so the area behind the wall is not drawn and fps is not significantly affected).
*where an area is a convex shape between structural brushes.
