Vandatory
When something is deemed voluntary, yet necessary. Thus, it could technically be thought of as mandatory despite being voluntary.
Person 1: "Man, college is going to bite me in the ass. Maybe I just shouldn't attend."
Person 2: "Yeah, college is vandatory like that."
Person 1: "What do you mean by that?"
Person 2: "Well you don't have to go to college, but good luck with life if you don't."
Person 2: "Yeah, college is vandatory like that."
Person 1: "What do you mean by that?"
Person 2: "Well you don't have to go to college, but good luck with life if you don't."