Semantics: "VDB"
 

Does the phrase "Voluntarily Denied Boarding" make sense?

"Denied" implies that you requested to board, then were refused. But in reality you never asked to board the plane; in fact, you agreed not to.

Similarly, is the phrase "IDB" / "Involuntarily Denied Boarding" a tautology?

Sure it makes sense. You volunteered to not go. IDB is you did NOT voluneer. ;)

Right, you volunteered not to go. So at what point were you denied boarding?

My guess is that after IDB came into existence, the concept of VDB came into use, and the phrase wasn't fixed grammatically for consistency, i.e. the voluntary version of DB.

The terminology is consistent with that used in the U.S. Code.

PART 250--OVERSALES--Table of Contents
Sec. 250.2b Carriers to request volunteers for denied boarding.

It has to be called denied boarding because they sold confirmed space to more passengers than they had seats for. Voluntary or involuntary refers to the selection process of how they chose exactly which people get denied.