I added urea to water and, predictably, observed a temperature drop (10 C). I wanted to place the beaker under (reasonably) high vacuum (for a classroom) and recover the urea as a dry substance. Well, I didn't. It's been a couple of months now, and there's white scale all the way up the inside, and crystaline filaments on the bottom, with an unchanging fraction of the original H2O. Is the substance still urea? There's been no odor. Is it ammonium carbamate? In either case, can I use vacuum to dessicate it?
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.