The Scorpion isn't a pistol. Don't treat it as one. The Scorpion is a pistol-sized grenade launcher. Used appropriately, it is remarkably fun.
The Acolyte is marginally more annoying to use because of the charge-up, but it is the single best shield/barrier stripping weapon in the game. The bouncing of the projectile can be difficult to predict, but it allows for the occasional trick shot of hitting something behind cover, and direct hits will eliminate shields.
I agree with you about the gimmicks.. at least until you learn to use them appropriately. I have no use for the Talon merc, but the Krogan Warlord is basically a walking explosion. Throw him into groups and unleash the hammer, the shockwave of the explosion will decimate spawns.
Yes, the Slayer is basically Kai Leng with a helmet, but he's also a Vanguard with a ranged Nova. Thematically, you'd use swords primarily to get around shields. In the lore, shield barriers are very good against bullets, not so good against low-velocity fists, omniblades, and other melee weapons. Unlike Kai Leng, however, the sword is NOT your primary weapon as an N7 Slayer.
The N7 Shadow is based on the Phantom, certainly. And Shadow Strike gives you amazing mobility, much like a Vanguard though more focused.
The Turian Havoc is a tech-based Vanguard with a great deal of battlefield mobility. USE that mobility to attack and put yourself in a position where you can put enemies in a crossfire with your teammates.
The Volus Protector is a great kit. Definitely one of the better Vanguard kits. It's only your perception that the Volus Protector is meant to be more of a joke than the other Vanguards that leads you to believe you should be more effective with something else. The Volus has *two* shield recovery mechanisms; Charge and Shield Boost, and it has a power recharge improving ability with the Biotic Orbs. Orbs that also do damage and debuff. It's going to be very survivable and a very high damage character to boot.
The key with the Graal is to be accurate with it. Use the charge up only when you can take your time aiming. You will need a bit of distance, trying to get in close and personal means you will miss more often than not. However, I've seen it take down a pair of enemies that were directly in line with the shot, so it's got a remarkable amount of penetration.
And the Striker.. look, like the Talon Venom, the Falcon and the Scorpion, the Striker is a grenade launcher. A full-auto grenade launcher, in this case. While it can be used to score direct hits, it's main utility is in the fact it flings grenades. Again, use it like one.
Frankly, I'm not looking forward to the Vorcha. Any of them. Losing control of the character, and popping out of cover for a couple seconds while that damn rage animation goes off is a guaranteed killer. And you *need* that rage for the regen and damage boosts.
The Volus Sentinel. Sure, it's a very effective kit. But it's boring. Send drone into spawn, run around shooting stuff with an SMG, throw down a decoy to.. well, uh, I guess it does *something* but by the time it dies and explodes, the spawn around it is already long dead thanks to teammates.