Do plants benefit from the vitamins they produce?

Oranges as not the plants that produce the most Vitamin C but they do well. The high vitamin C may protect the seeds from predators.

My very basic understanding is that the fruit develops to protect and then nourish the seed once it has ripened and fallen to the ground and the fruit decomposes into the soil.

Not only do they benefit from them, it is essential:
http://www.sciencedaily.com/releases/2007/09/070923205844.htm

Plants benefit from minerals found in the soil such as nitrogen.

@ragingloli , I wonder if the function in plants is in some sense similar to that in animals.

Yep.

Take your example, for example.

Because they’re sucking up all that vitamin C in their fruit, orange trees never catch colds! ;-o