Just as a hard-follow-up-week trying things here and there, I finally made a custom vector work.
Unfortunately I did it with (another) direct light. Both light sources (sky light) and my custom (object) direct light competed, and cancelled out the clouds in the (default third person project) sky.
I figured I could set the Forward Shading priority to 5 (main sky light) and 0 to my custom (light object - direct light), but I have no result on the sky.
(Custom vector solved). Sorry, I did not understand anything about what you mentioned about the material producing the light in your previous reply.
I solved the custom light vector on my own , but I created another problem with lights.
(EDIT): I inadvertently chose “unlit”. Back to LIT: Default lights, everything runs great. Custom vector + scene lights work.