Evolution I
Incarnon Mode is a 'Fan the Hammer style shot with 3 4-burst shots. You cannot zoom while firing in incarnon. The -100% Recoil we obtain in the build means you won't miss if you've got your aim right.

Evolution II
Lone Gun is the keystone evolution that makes this build work. You must lose access to primary Weapons in exchange for extra firepower.

Evolution III
Extended Volley - Extra Ammunition if you miss your headshots
Marksman's Hand - If you intend on using Spry Sights you need this to combine with Secondary Deadhead to achieve -100% Recoil

Evolution IV
Commodore's Fortune - if you want more Critical Hits or Reaper's Plenty for not using ammo while you're popping enemy heads.
Reaper's Plenty - Transforms the Vasto in a Faux Knell letting you fire non-stop as long as you're hitting the enemy in their heads.

[Creeping Bullseye] can be swapped out for [Galvanized Shot] if you want more status procs.
For Corpus swap Ice Storm for Expel Corpus & Primed Heated Charge for Pathogen Rounds

Exilus:
Spry Sights - Has synergy with Galvanized Crosshairs giving you more movement while aiming
Steady Hands - Removes recoil entirely if you combine with Secondary Deadhead or Marksman's Hand

Arcanes:
Secondary Deadhead - Synergizes with Crosshairs to pump up your Critical Damage and additional Recoil Reduction

Warframe Arcane:
Arcane Precision - Headshots grant increased Secondary Weapon Damage you'll be constantly going for headshots so this is a flat damage boost.