The question of the lack of alien life in the Dune universe has been asked many times. The coverage of the Known Universe is still small enough, in the massive scale of the universe that the Fermi paradox does not hold. But that’s not the reason there are no aliens in Dune.
The Vicinis are a military arm of the Spacing Guild. They’re forbidden to act officially on the planets of the Landsraad. The Vicinis Agents are tasked with “clearing the way” and “keeping the borders”.
The Vicinis are organised like a military unit.
Their craft are much smaller than Heighliners, but they are still immense and crewed by a Navigator, as all ships that fold-space must.
But these are made for war.
What have they done?
The Vicinis should be considered a little like “Colonial Marines”. Their mission is simple – keep the borders of the known universe, of the human centric universe, clear of alien influence.
Through prescience, the Vicinis will fold space to hitherto uncontacted planets and systematically wipe out anything that resembles intelligent alien life. They will do this ferociously, they will do this without quarter or mercy. They’ve done it before, they’ve done it a thousand times.