Billy had no idea how well Vecna impersonated him, or if Eddie had encountered him at all.

When Eddie taunts him, cants that hip, Billy storms over to grab the freak by his shirt, slamming him to the door he came through. "Better think twice on that, Munson, before I make you as my next victim." Now that sounds more like something Vecna would say, but it's all Billy, sneering and ready to lash out at the easiest target here.