You might be able to get Vastnesses after summoning 20, but there's a "might" here (I think it might be possible to get them a few levels after 20, but they will completely disappear afterwards).
This part was actually confirmed by one of the developpers if memory serves, though they only said the odd of getting a Vastness decreased as summoning level increased, and not that it was impossible to get them past a certain threshold.
But yes, if you do want Vastnesses, you should switch your summoners after level 20 or so; then again, Greater Othernesses aren't the worst thing in the world either.
