If one had a light course of encephalitis before, is the vaccine against it still needed? or the organism develops antibodies?

More than one cause: Encephalitis means inflammation of the brain. There are many viruses, bacteria and other organisms that can cause it. If you recover from a mild case of viral encephalitis you are likely immune to that virus but not to a multitude of other viruses. Even with a seemingly single virus, e.g. Dengue, there may be multiple strains and infection with one may not protect against others.

Please clarify: There is no vaccine for encephalitis, assuming you are referring to the viral type. There are vaccines for meningitis. Generally when one gets an infection, one does develop antibodies to fight the disease in the future. The organism does not develop antibodies, your body does. And that is how you get protected. So you need to specify what type of encephalitis to get a clear answer. Hope I have explained it well enough.