It really depends on what you are doing at the time. Pits: Best choices here are Fatebringer, Suros Regime or the Vex Mythoclast. Fatebringer may only be a legendary but it has very reliable explosive rounds enabling you to clear out a large number of thralls with minimal effort. Suros Regime because of it's ability to clear out the thralls with a small chance of healing you at the same time. The Vex is basically the same as the Suros but does more damage without the healing factor. I would advise against using the MIDA unless you have a hard time staying with your group or have enough survivability to run ahead and pop thralls before getting to the light. Or if your entire team has them ;) Bridge: Pretty much whatever keeps you alive. Icebreaker to help out on opposite sides of the bridge, Gjhally for Swordbearers and whatever Primary you're comfortable with. Deathsinger: If you're pulling the Wizards, go with the Vex. If you're dp then Icebreaker. For the Shriekers, I'd go Icebreaker unless you have a lot of heavy ammo. For the singer herself, all Gjhally all the time :) Don't forget about that 3 min time limit. Crota 1st Stage: For the opening battle, Vex, Suros or Red Death. Gotta love dem healz :D 2nd Stage: This stage begins when you start fighting Crota proper. For Boomer (Knights) duty, Icebreaker is really your only options with a nice Legendary to assist with clearing adds and dropping Swordbearers. For Swordbearer duty, all Gjhally all the time.

I recently acquired a Vex and am wondering the same thing too. I feel like the higher attack value will be more useful in hard mode but it's hard to give up the Gjallarhorn, even with a 331 Hunger of Crota available.

I think the vex is great for the first section, after which I switch to VoC/icebreaker or VoC/Gjallahorn. Assuming that VoC will not be too underpowered in hard mode I imagine my strategy will remain the same as it has been for normal.