I think it has to do with the updates somehow.
One just needs to refresh from another vegvisir and it should be there.

The one I summoned last night was stuck at the top of one of the indestructible trees

the vegvisir points to where the moder pedestal will spawn at the moment you view it. Since it doesn’t actually spawn it, only puts a map marker to where it will spawn, when the game updates in a way that updates generation (like the frost cave update) the generation changes and the spot that before would have spawned a moder pedestal now doesn’t. You’ll just have to reuse the vegvisir to update the location