Image: UbisoftOfficial screenshot from Ubisoft.
This Vaas: Insanity trophy asks you to unlock every weapon in Vaas' Armory. It only counts in a solo game, and it takes several runs of steady progress.
Clear the Gun Shows for every weapon type on the map. Each one opens its Weapon Case at the Armory. Spend Cash on the cases until every weapon is unlocked.
Because Armory progress is permanent, spend spare Cash on cases before a risky fight. Focus on one weapon type per run if money is tight.