This study presents a novel chemical reduction method for producing high-quality reduced graphene oxide (RG-O) using hydriodic acid (HI) and acetic acid (HI-AcOH) in both solution and gas phases. The method enables efficient, one-pot reduction of solution-phase RG-O powder and vapour-phase RG-O (VRG-O) paper and thin films. The resulting RG-O, denoted RG-O$_{HI-AcOH}$, exhibits high conductivity, low-temperature processability, and excellent dispersibility in various solvents. The reduction process effectively removes oxygen functional groups and enhances graphitization, leading to a significant increase in electrical conductivity compared to other reduction methods. The RG-O$_{HI-AcOH}$ produced in this study shows a high C/O ratio, minimal nitrogen impurities, and a well-ordered two-dimensional structure. The study also demonstrates the fabrication of flexible VRG-O papers and thin films on plastic substrates, with the VRG-O$_{HI-AcOH}$ paper showing significantly higher electrical conductivity than the VRG-O$_{NH_2-NH_2}$ paper. The method offers a promising approach for the mass production of high-conductivity graphene-based materials on flexible substrates. The results highlight the potential of HI-AcOH as a superior reducing agent for the chemical graphitization of graphene oxide, leading to the development of high-quality RG-O for various applications in electronics and optoelectronics.This study presents a novel chemical reduction method for producing high-quality reduced graphene oxide (RG-O) using hydriodic acid (HI) and acetic acid (HI-AcOH) in both solution and gas phases. The method enables efficient, one-pot reduction of solution-phase RG-O powder and vapour-phase RG-O (VRG-O) paper and thin films. The resulting RG-O, denoted RG-O$_{HI-AcOH}$, exhibits high conductivity, low-temperature processability, and excellent dispersibility in various solvents. The reduction process effectively removes oxygen functional groups and enhances graphitization, leading to a significant increase in electrical conductivity compared to other reduction methods. The RG-O$_{HI-AcOH}$ produced in this study shows a high C/O ratio, minimal nitrogen impurities, and a well-ordered two-dimensional structure. The study also demonstrates the fabrication of flexible VRG-O papers and thin films on plastic substrates, with the VRG-O$_{HI-AcOH}$ paper showing significantly higher electrical conductivity than the VRG-O$_{NH_2-NH_2}$ paper. The method offers a promising approach for the mass production of high-conductivity graphene-based materials on flexible substrates. The results highlight the potential of HI-AcOH as a superior reducing agent for the chemical graphitization of graphene oxide, leading to the development of high-quality RG-O for various applications in electronics and optoelectronics.