I don't want to spoil the trick for those who don't want to be spoiled, but I also don't want you to feel like I'm playing some sort of juvenile game of keep away. So, I'm going to put the thing about the island here in an sblock. It's the same information that you'd get if someone succeeded on an Idea roll at this point. Look if you want; don't look if you'd rather figure it out IC.
John sees the island as there. Vernon and Wink do not. You're all sharing one set of eyes - as Sig said. As long as people who see different things are looking, you're going to have a problem. If John shuts his eyes, the experience of the island being present goes away. If Wink and Vernon shut theirs, the experience of it not being there goes away.
Because you're a smart person, you're probably thinking that there's a broader problem with this, and you're right. That's part of the Mythos horror of this adventure - the implication and potential for broader disruption if some people experience the island while others do not. What would happen if John (or someone else who could see the island) happened to be looking out to sea when a liner passed through the space where the island is and isn't?
Is it just the island, or are there other things that are both there and not there that some people experience and others don't?
Why is John different? Why was Sig? Or the nurse who saw the island?