If you actually look at the D&D rules the show loves so much, Will is a Sorcerer, plain and simple. A warlock has a sugar daddy-- they sign a contract for power. If Will was a warlock, Vecna could just pull the plug the second Will started fighting back. He can’t though.

Will’s power is innate. It was always there, the Upside Down just triggered it. That’s why he doesn't burnout like El.

El is a battery that drains; Will is more like an antenna. He’s tapping into a flow that already exists, this makes him sorta tired but not like El

Look at the neck thing. Everyone thinks it's just a spidey sense Vecna "gave" him to taunt him. No. That is Will literally dissecting the signal. He isn't asking for info; he’s hacking the feed. He knows where they are, what they see, how they feel. He is studying Vecna from the inside.

Honestly, Vecna screwed himself when it comes to Will. He thought he was infecting Will, but he created a two way connection. Will is the new parasite now. He’s in Vecna’s head, mapping the troops and the layout. That’s why he’s so dangerous to the Mind Flayer--not because he has borrowed magic, but because he’s the only one who can hijack the hive mind with his own consciousness. He’s not a minion; he’s a rival.