Graphene based vertical organic field-effect transistors (VOFETs) are promising devices for realizing a high current density at a low operation voltage. However, high-quality graphene typically requires a high temperature and a complicated fabrication process, presenting a major barrier to roll-to-roll manufacturing. Here, we report a low temperature (⩽45 °C), vapor (HI)-assisted process to achieve ultrathin and uniform reduced graphene oxide (rGO) films with a higher electrical conductivity compared with conventional high-temperature reduction methods. Notably, VOFETs with rGO source electrodes exhibited the highest current on/off ratio exceeding 10⁴ and a maximum current density of 14.4 mA cm⁻² owing to the gate-tunable work function characteristics of the prepared rGO. The current density of the device could be further improved by increasing the conductivity of the rGO electrode with additional thermal treatment. The synergy of low-temperature and HI vapor endowed rGO-VOFET devices with excellent current density, on/off ratio, and gate-tunable ability, making them promising candidates for future flexible organic electronics.