The best way is to install a breather vent/vacuum valve on the tank but you have said you can not take any pressure in the tank (have you attempted to go back and rate the tank for pressure and vacuum, you only need a few inches of water to use a vent/vacuum valve?) The tank will still vent even with a valve when it fills and displaces the equivalent vapor volume but you won't lose water vapor when the tank is stagnant.
If the vent is open to atmosphere and you have to leave it open, the only thing that comes to mind is to install some sort of condenser coil in the vent line that will condense the water vapor and return it to the tank (there won't be any significant amount of glycol in the water phase).
Why does the fluid get up to 50C, can you change this? At 50C, the vapor pressure above the fluid of water is going to be much higher than at 15C and that is directly attributing to the losses. The cheapest solution is going to cotinue to make up water to the system every so often to maintain concentration.
