The internal configurational entropy of point defect clusters in crystalline silicon is studied in detail by analyzing their potential energy landscapes. Both on-lattice and off-lattice calculation approaches are employed to demonstrate the importance of off-lattice configurational states that arise due to a large number of inherent structures (local minima) in the energy landscape generated by the interatomic potential function. The resulting cluster configurational entropy of formation is shown to exhibit behavior that is qualitatively similar to that observed in supercooled liquids and amorphous solids and substantially alters the thermodynamic properties of point defect clusters in crystals at high temperature. This behavior is shown to be independent of interatomic potential and cluster type, and suggests that defects in crystals at high temperature should be generally described by a quasicontinuous collection of nondegenerate states rather than as a single ground state structure. The modified thermodynamic properties of vacancy clusters at high temperature are found to explain a longstanding discrepancy between simulation predictions and experimental measurements of vacancy aggregation dynamics in silicon.