Maybe you find the FEV strain in the abandoned lab where you find Virgils Cure, you could have the PC get attacked and bitten by a mutant that roams the lab and get knocked out(fade to black when you activate the FEV strain vial). Upon waking up the PC will hear the aforementioned voice in their head, and has an empty FEV vial in their inventory. You could then have super mutants and ghouls appear with unique powers, and force the pc to hunt these creatures down and loot them in order to gain the powers. Similar to the way Automatron works, where the robots only spawn after the quest starts, and you have to loot em for parts.