Now that you know what you’re looking for, putting another spell in the spine should let you follow the link, assuming the wizard takes the bait. It seems worth a try, at the very least.
You cast another spell, and the insects are drawn to it, but you pay them no heed. Instead you watch the ether.
There’s no sign of a link, but now it’s clear to you what’s going on – the wizard is inside the spine. You were ejected rather than transformed because they didn’t want you to find them, and transforming you would have been more difficult to do quickly.
But shouldn’t Vasco have seen them, then? He’s ether-sensitive, and a wizard doing this would have to be in etherform. Unfortunately, it’s going to be difficult to ask him questions about that without tipping off the wizard, assuming they’re using the spine to listen.
They could have hidden in the spine, but you were in it yourself. There didn’t really seem to be anywhere to hide. So if they did slip by Vasco, how might they have done it? Perhaps you can use their own trick against them somehow.
Come up through the floor. Perhaps concealed inside something like a bug