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The fact that the thermoelectric performance is far inferior to that of p-type PbTe has inspired many strategies to develop n-type PbTe thermoelectrics. Alloying PbS in n-type PbTe effectively changes the shape of a valley to trigger a heavier conduction band for improving the Seebeck coefficient, while the resulting small orbital overlap inevitably leads to phase separation hindering electron transport. The effect of vacancies on the solubility of sulfur in n-type PbTe is ambiguous; especially, the heterostructure due to phase separation in high-content PbS-alloyed PbTe also requires sufficient modification to optimize the electroacoustic transport. This motivates the current work on the introduction of vacancies by charge-balancing doping via Sb2Te3 and discovers striking new insight that the introduced vacancies can induce a new heterostructure of Pb2Sb2S5 and suppress the aggregation of Sb and PbS in high-solubility n-type PbTe–PbS. The modification of the band structure and optimization of the electron transport give rise to a prominent enhancement in electronic performance. Furthermore, the Debye–Callaway model validates the dramatic contribution of vacancy aggregation and heterostructures to lattice thermal conductivity. As a result, the synergistic modulation of electroacoustic characteristics achieves a significant improvement in both the maximum zT and the near-room-temperature zT. Understanding such unique findings is critical for applicability to other thermoelectric materials.