Do airplanes have vapor recovery systems for refuelling?
You are correct on both your assumptions about the fuel vapour. It does have a greenhouse effect, though I would suspect that the amount of fuel required to make the air saturated in the fuel tank is not likely to be more than some tens of grams. It also presents a fire hazard.
However we again face the same issue as with many other systems that would be environment-friendly and safe: the cost.
Companies, both airlines and airport operators, carefully evaluate all cost to benefits. In this particular case the environmental effect is likely to be considered neglible, and the fire risk is tackled with other measures, such as grounding, clear-areas, ventilation etc. Those were covered nicely in the previous questions.