Lanthanum cerate (La₂Ce₂O₇, LC) is a promising thermal barrier coating (TBC) candidate with superior thermophysical properties over yttria-stabilized zirconia (YSZ), but its practical application is hindered by low-temperature thermal expansion coefficient (TEC) contraction. Previous studies primarily focused on regulating the oxygen vacancy concentration while neglecting the influences of vacancy distribution. Herein, we employ Sm³⁺ isovalent substitution for La³⁺ to maintain a constant vacancy concentration and isolate the vacancy distribution effects. The optimal composition (La_0.8Sm_0.2)₂Ce₂O₇ significantly suppresses low-temperature contraction, reducing the linear shrinkage rate by ~88.5% and increasing the TEC by ~12.56% (to 12.37×10^–6 K^–1) compared with LC (10.99×10^–6 K^–1). Combining density functional theory (DFT) calculations and HR-TEM/AC-STEM characterization, we directly reveal Sm³⁺-induced oxygen vacancy clustering in LC-based ceramics. The underlying mechanism involves (i) randomly distributed free vacancies inducing contraction via vacancy-phonon coupling and local symmetry breaking; (ii) Sm³⁺ substitution introducing dislocations whose stress fields, together with Sm³⁺’s higher ionic potential, drive vacancy clustering; (iii) clustering reducing mobile vacancies and restoring lattice order, thereby suppressing contraction. This work confirms that LC's low-temperature contraction is coregulated by vacancy concentration and distribution, complementing existing concentration-modulation strategies.