It is very important to cool the cooling liquid running through your rotary evaporator's condenser. Unless you have a dry ice condenser or an in-house cooled water circulating system you are going to need a chiller for this purpose.
Chilling the cooling liquid will ensure that solvent vapors are condensed and can be recollected, properly disposed of and/or recycled. Condensation also prevents the potentially toxic solvent vapors from being pulled into the vacuum pump and eventually being released in to the atmosphere.
On top of that, solvent vapors can contaminate pump oil, degrading the pump’s performance and lowering the oil’s boiling point. This can result in the spread of fine oil mist and sometimes bumps of oil spilling over. Depending on the solvent it can also potentially cause serious mechanical damage to the pump.
To avoid this and ensure a safe working environment always make sure that the cooling liquid running through the condenser is cooled at a temperature of at least 20°C cooler (rule of 20) than the vapor temperature to ensure full condensation of all vapors.