Spawn monsters are generally above other effects, like Monster Limits and gate closures. But in this case, I think they would go through the Vortex and end up back on Glaaki. Follow the procedure as if they were normal Monsters.
However I think this should also apply to cards, spells, etc. that tell you to return monsters to the monster cup.
It has been argued that these effects would not affect spawn monsters, but I disagree.
Imho, whenever an effect tells you to return monsters to the monster cup it will affect spawn monsters but they are returned to the AO instead.
As Mrs Gamura pointed out, the cards that say to remove monsters has specifically been answered by Kevin. Spawn monsters are only removed if defeated.
The only reason that I suggest Vortices are exempt is because they create a situation that would require even more clarification. If they get stuck, then there is no defeatign them whatsoever. And they are meant to be defeated. And since Investigators can't enete a Vortex, then they would be in a permanent sort of Outskirts.
But on the other hand, maybe Kevin intended them to get stuck in Vortices and therefore harkin Glakki to awaken that much quicker. This may be the preference since Servants of Glaaki and Vortices were introduced in the same expansion, I have to assume this was encountered in playtesting (it's not like two features from two different expansions that might have been overlooked). So it's very possible that they are meant to get stuck, bring about the awakening of Glaaki that much faster, and pump up the Terror track.
So I can see a case made either way, I think it would just be "cleaner" to follow the first point. But that is just an opinion.