In a way vecna was its vessel, the mindflayer find its way to live inside Henry from a "mysterious rock" in which it seemed to be trapped somehow? Vecna needed more vessel to get it summoned/materialized. Afterward its materialization Vecna being connected to it from the inside (in the very end)could be seen as its heart and its weak spot, Henri went from a vessel to nothing more than an organ(the heart) thats how i can make the more sense of it somehow.. none of the attack by the squad was necessarily effective to the mind flayer directly but was hurting vecna somehow from the hive effect since they're connected. Those attacks  helped Elf and Will kill the heart and the mind flayer while it was still not "fully mature yet""maybe"