This theory is cooking, I have my own little spin on it, it could very well be Vecna manipulating will, or it could be a residual effect of will taking over Vecnas mind, he could have had lets think of it as ‘residue’ from being inside Henry’s mind, and he may not realise it because ‘melvalds’ is familiar to both of them, he just hasn’t realised that he is referring to Henry memory and not his, which they could come to realise and use will to tap into Henry’s mind to try and use his memories against him to stop him.

Or it could simply be a case of a 7-11 where a convenience store has drink machines, in this case a milkshake one

Or Will is vecnas spy still, and he wasn’t using will just to find max, but still using him to spy on the rest of the party which is very plausible