Superman is not invulnerable to magic.
He is just as vulnerable as any one else.
Though this is also inconsistent in comics.
Let me give a couple of examples
In Invasion he got "affected" by magic from a very powerful voodoo priest and a scan with out context will indicate that Superman just got defeated by deadly voodoo magic designed to kill him.
Bear in mind that the Voodoo Priest on panel can kill with his deadly voodoo magic.
But if you read the comic, you will realize that before the voodoo priest strikes he mention something about Superman's weakened mental state because of his mental break down followed by the execution of the pocket verse kryptonians AND the fact that he just got blasted by a Tannagarian cannon capable of disentigrate a small ship.
So out of context it seem that Superman just got defeated by deadly Voodoo magic, but with context you realize that the "effectiveness" of the deadly voodoo magic was only partial and due to extenuating circumstances that allowed the Voodoo priest to KO Superman.
1.- Deadly Voodoo magic, confirmed to be deadly on panel to a regular human
2.- Weakened Mental state (that lower Superman defenses)
3.- Blasted by a Tannagarian Cannon that can desintegrate a small ship
So being as "vulnerable" to magic as anyone else will depend on the writer.
As far as I remember the only time that Superman showed a "weakness" to magic was when he got shot with a Voodoo bone bullet as Plastic took the same bullet but it did not affected him and IIRC it was explained that Plas got an specific advantage because of his ability of changing his molecules or something like that.
He has also survived a magical blast that according to narration will "rip a tank in two"
in short Magic is not a weakness but a vulnerability IF you have enough magical juice, IMO