Will a white sony VAIO laptop (the one with luminous coating) ever turn yellow after long time of usage?

I don't think the Vaio laptops have been out that long yet. That color should not be changed/turned to yellow.

yes . its due to the sweat your wrist produces during typing. Of course if you use a suitable disinfectant (that wouldnt harm the material, say so strong that it removes the white colour) to remove the sweat , the computer will remain at a well condition for longer period of time . BUT the whiteness will still fade off.

possibly if you dont take care of her well