It depends on the raid and the player's skills IMO. I'm far from an expert, but here's my experience after playing both raids several times with all classes and subclasses. In the Vault of Glass, I feel that voidwalker warlocks are the primary heavy damage dealers in the team with nova bomb, to wipe out fanatics during the legions phase when defending the confluxes, or to help kill the praetorians and such when teleporting to Venus during Atheon. I think they're fantastic during the lamps section on Crota's End as well, especially with the Obsidian Mind exotic helm, nova bomb can kill big packs of thrall easily and recharges very quickly with the bonus from Obsidian Mind. Conversely, and controversially maybe, I think sunsinger warlocks have no real purpose in either raid other than to serve as a spawn point with fireborn to save the team from getting wiped. At least that's what most sunsingers use radiance for IME, I do it too, and it does have its place (relic holder in the Vault, for example, should something go wrong and you die, you can self res and pick the relic right back up before your team dies). Or they can act as sort of a team medic, running out somewhere you wouldn't normally go to try and revive a teammate, and if they get killed in the process they can self res and usually get the dead teammate up as well before radiance wears off. I've tried using Song of Flame to reduce cooldown for my team, we never really noticed any huge benefit from it, but YMMV. The sunsinger's grenades are pretty good though, Gift of the sun + Sunbreakers can be useful for area denial in the lamps section of Crota, solar grenades do a good job of blocking the path behind you as you're running away from thrall. Firebolt grenade + touch of flame works well at killing lots of thrall too. Hunters in the Vault of Glass are great for ad control, gunslingers with chain of woe/gunslinger's trance are great for getting a lot of kills on cannon fodder goblins or whatever. Golden gun is great for dealing with praetorians on Venus during Atheon, it's good for killing oracles too. I don't think Bladedancers are all that useful in the Vault, mostly because I don't think arc blade is super useful in most circumstances. Maybe clearing a conflux in a hurry, but even then if there's a fanatic somewhere in the mix the bladedancer is probably gonna get marked and maybe killed by the fanatic's explosion. Bladedancers are handy in Crota's End though, arc blade is great during the lamps section for killing thrall - especially with the encore talent and the bonus from Mask of the Third Man - and stalker + shadowjack lets you be sneaky if you wanted to, jumping up on the platform where Crota is before he's actually down so you can maximize the licks you put on him with the sword. I feel for all of the Vault of Glass, and maybe for the last bit of Crota's End, Defender Titans are the preferred subclass of titan, they're your support guys. Ward of Dawn/Weapons of Light can help make boss fights shorter and gives you some temporary cover if needed. Plus with the Saint-14 helm it basically gives you full coverage for a few minutes as every enemy that enters the bubble is blinded and useless, giving you time to either kill them, revive dead teammates, run further away to recover, etc. They're very powerful in the Vault, magnetic grenades wreck praetorians, suppressor grenades are great against wizards in Crota's End. They're hearty meat shields which make them good relic holders or sword swingers as well. Titans should be running the relic in the Vault IMO, mostly because of the meat shield thing, and their supers aren't all that effect at killing stuff during Templar (I guess you could use Fist of Havoc on harpies if you really wanted to, but Weapons of Light against the Templar is more useful for everyone on the team) or teleports during Atheon (titan slamming a praetorian is probably a bad idea though). They can also jump the highest, which can help during the Crota fight since they generally have an easier time getting up to the platform from dead center or to when Crota is to the right or left than a warlock or hunter does. Striker titans are great for the Crota lamps section, Fist of Havoc kills a lot of thrall in a hurry. Flashbang grenades make dealing with the gatekeepers during the bridge section a piece of cake. And using transfusion or with an upgraded Insurmountable Skullfort helm, you can keep yourself alive during the Crota fight by punching thrall here and there to regen health. So yeah, several different roles depending on the player, subclass, and particular raid/situation, IMO.