Urea + water yields ... urea + water?
As water evaporates, the vapor pressure above the more concentrated urea solution drops. Either a better vacuum pump is needed, or perhaps gentle warming, to extract more water.

You might also try chilling to (partially) separate the solution. At ~260 K, achieved in many home refrigerators, much of the urea will have crystallized from the remaining saturated solution... just don't mix this up with food in the freezer.