Water is the ideal fluid for a vapor chamber. Other fluids like isopropyl alcohol, ethanol, methanol, acetone, etc. have been tried but found wanting, with lower heat storage/transport capacity. Basically, water's latent heat of vaporization is very high, at about 2257 kJ/kg, so it carries a lot of heat per gram. Its specific heat capacity is also very high, at about 4.18 kJ/kg·K, so it absorbs heat better before boiling. All those other fluids have much lower figures. Their boiling points are also too low, so they boil too early, or too high, preventing them from boiling at phone temps. There's also safety concerns with some of these fluids (toxicity, flammability, and/or corrosion), some are more costly than water, and some degrade or leak more easily.