Morphological control and surface vacancy modulation are effective strategies for promoting the readiness level of g-C3N4-based photocatalytic systems. In this work, we present an unprecedented proof-of-concept study on the doping effect of nitrogen vacancies in g-C3N4 atomic layers for enhanced CO2 photoreductivity. A 3D bubbly architecture scaffolded by few-atomic-layer (ca. 1.6 nm) g-C3N4 nanosheets with the potential to facilitate charge transportation was successfully realized by inducing a disruption in the π-π interactions between adjacent layers of g-C3N4 through simple incorporation of an NH4Cl gas template. Introduction of nitrogen vacancy defects into the ultrathin nanosheets further exerted remarkable tuning effects on their optoelectronic properties. The concomitant outcomes from the event of nitrogen vacancy introduction gave rise to several phenomena, including: (i) an increase in reducing ability with a higher nitrogen vacancy density, (ii) extended absorptivity to the long-wavelength visible light region due to the strategic band position of the midgap state below the CB, and (iii) prolonged radiative recombination of photogenerated electron-hole pairs, as the midgap state serves as an effective reservoir to temporarily trap electrons. As a result, the photoreduction performance of nitrogen defect-modified g-C3N4 atomic layers was synergistically improved with 3.16-fold and 5.14-fold enhancements in the CH4 evolution yield over their respective pristine ultrathin counterpart and bulk form.