A thermosyphon vapor chamber was manufactured and tested. The evaporator is made of copper and has a circular geometry (185 mm outer diameter). The condenser consists of an array of hollow fins made of 300 µm thick polymer layer and having an inner diameter ranging from 5.3 to 10.7 mm. It is cooled by forced convection. Experiments were carried out to characterize operating regimes depending on the working fluid, fins diameter, fluid fill charge and heat flux. HFE-7100, n-pentane and water were tested. Visualizations were made, as well as temperature and pressure measurements over time. For HFE and pentane, a stable oscillating regime was observed for all operating conditions, with film condensation and nucleate boiling. For water, dropwise condensation occurs and many hydrodynamic phenomena were observed depending on the operating conditions (geyser effect, liquid plugs, flooding of tubes), leading to intermittent behaviors. Operating regime maps are proposed. The influence of the regimes on the thermal performance of the vapor chamber is small as long as the fin inner diameter is large enough to prevent the occurrence of stable liquid plugs occupying the whole height of the tubes. The determination of the critical diameter is thus of great importance for the performance optimization, considering condensation heat transfer as well as external convection.