Cooling would probably be an issue in vacuum - the primary cooling channel for freshly deposited plastic would be by transfering heat to the layer below it. The first few layers might not have an issue, but I suspect that as it got thicker the rigidity of the print would suffer dramatically, especially if there were any narrow choke points restricting the heat flow. It'd be like trying to print with jello after a while. You could slow the printing to allow adequate thermal dissipation, but that seems counterproductive.
You might also have other issues - for example what's the boiling point of plastic in vacuum? If nothing else those toxic gasses we're trying to avoid are going to escape from the plastic much more energetically if there's no ambient pressure.