You have to do between 100-300 activities (strikes, crucible, vex offensive) with the gun equipped just to obtain the catalyst. Even 100 with both boosters is a bit much
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They’ll adjust the requirements just like they did for the mountaintop quest. It’ll take a while but this is what happens when Bungie doesn’t play their own game. I did the mountaintop quest before the adjustment, so I learned my lesson and will patiently wait for the Vow adjustment.