It's a fairly obscure chivalric order which pertains specifically to the tensions between European Christianity and the Ottoman Empire. The order diminished somewhat with the death of its founder, Sigismund of Hungary, but remained regionally active along the border between the Ottoman Empire and Christian Europe.

Vlad II Dracul was a member of the somewhat less prestigious group of inductees to the order in 1431 and the honor passed to his son, Vlad III Dracula (of sanguinius fame).

The major reason you don't hear more about the order is that it was always a pale imitation of more prestigious and politically important orders founded around the same time, typically having to do with crusades or other religious conflicts.

Wikipedia has a decent list of members though the only names besides Vlad II Dracul and Vlad III Dracula that you are likely to recognize is Henry V of England, counted only as an unsworn "foreign ally."

For more on Vlad and his delinquent son I recommend In Search of Dracula which isn't focused on Vlad specifically but does give a wonderful treatment of the historical foundations of the Vampire myth, Vlad included.