From Page 246 of the hard cover: “As Vhalantru notices them, he turns to face the PCs. Pressed up against the convex pane of his enormous central eye, the PCs can see the horribly fat visage of some demonic creature with a mouth full of sharp triangular teeth.”
From reading the description it sounds like Vhalantru starts the encounter with his central eye open. This implies that his anti magic cone is covering the PCs in the surprise round.
Do you agree or disagree with me?
I would agree, but at that position it would not cover the entire room. If the character can move out of that area they could use their magic items. I found that keeping everyone in the cone at the bottom of the room as outlined in the books is kind of boring, so you might want to adjust accordingly.