I ran Thorn for a bit, but it ran out of ammo too quickly. Then, I just switched to a decent scout rifle and waited to get a Vision of Confluence. Scout rifles (IMO) are really best suited for most parts of the content based on the range you'll find yourself in. Honestly, It's not really your primaries that are even important on the Vault for most of the run. You want a solid shotgun for Praetorians as well as sacrifices. Specifically, you want Swordbreaker, but anything that has max damage and a decent range will do. Next, you want a solid machine gun for bosses; or for pulling the clutch if it comes down to it. Since it's easiest to damage Atheon in the bubble where you can't usually fire rockets, something like Thunderlord or Super Good Advice is good for quick damage and guaranteeing your ammo is used effectively. The benefit Super Good Advice gives you also is doing stupid damage to Oracles during the Atheon fight. The Oracles don't seem to always register as hits, so a ton of ammo ends up going back into the magazine; this allows a stupid amount of damage to be done before you even start to run low.