I am learning to make a graphical engine with OpenGL. I wanted to know, should repetitive operations be moved from the vertex shader to the fragment shader, since from what I understood the vertex shader is only run once per vertex?
For instance, when normalizing a vector for the light direction, since this light is the same in the entire vertex should it be moved to the vertex shader, instead of calculating it for every pixel? Is there a particular reason to keep it in the fragment shader?
If the calculation is exactly the same: yes, it should usually be more efficient to do it in the vertex shader than the fragment shader. Some situations where it might not be more efficient:
when drawing geometry that results in fewer shaded pixels than transformable vertices -- either due to dense geometry or extreme discards/occlusion. If this is the case, usually you would want to address it by switching to lower level-of-detail geometry or smarter geometry culling.
when doing the calculation in the vertex shader requires you to send more data to the fragment shader in order to use the calculation's results. Sending more data can be slower because it requires more memory manipulation and because the rasterizer needs to interpolate more "varying" values across each polygon.
For light calculations, specifically, be mindful that moving calculations from the fragment shader to the vertex shader can affect the quality of your rendering. Particularly, normalized direction vectors at each vertex can become shorter after "varying" interpolation, which can slightly darken triangle interiors if used directly without renormalization. And, of course, moving the entire lighting calculation to the vertex shader has even more drastic effects.
But how visible these effects are depends on the frequency of textures, the resolution of geometry, the size on screen, how far away the lights are, etc. -- in some cases, the quality/performance tradeoff may make sense.