It depends on your goal and how it's measured.
I would argue that it might be impossible for it to be universally "fair". But I can contend quite easily the it could be more fair than it is.
Most people misinterpret the word "fair" to mean equal to both sides, but in sports that is a trap because it's virtually impossible for it to be true. There are too many variables. A much more realistic way to interpret what is fair in a sports context is insuring that no side is afforded any undue advantages. You set an optimal baseline (every team ices a full/healthy squad within the cap) and you do everything in your power to ensure that happens without allowing it to become an advantage. (Having a roster that would be impossible within the cap)
This is really just the trolley question, is it not? By allowing Vegas to exceed the cap, you have now created an unfair situation for 15 other teams. In a scenario in which there cannot be universal fairness, this is a wildly illogical choice.
Another argument is one of practicality. In a scenario without perfect fairness, we may have to make as choice about WHO should carry the weight of any unfairness that does exist. To me, it makes the most logical sense that any disadvantages that cannot be remedied should be carried by those with whom the disadvantages originated. In this case, Stone oo the Vegas Team as a whole. It is HIS injury and THEIR player. If any party were to be obligated to suffer, it is them. Not because it's fair, but because it's the most fair in an unfair circumstance.
Of course this is all an aside to the fact that they all had a choice and decided in that moment to make the unethical one.