My history behind the Vermin race is that they are a stable mutation of humans (similar to Squats and Beastmen) that were part of the initial colonisation exodus from the Sol system in the early early years of the Imperium. The settlers in question headed to the Galactic East, out to the Eastern fringe which is today out of reach of the Imperial Astronomicon. Cut off from the Imperium during the Imperium's Age of Isolation.

Their mutation is a result of generations of living on a group of planets in the Eastern Fringe that were subject to particularly fierce warpstorms occurring very close to the planet, and raining down asteroids of warp-material (Warpstone) upon the planets. The net result is a somewhat similar to a human with a shorter lifespan (due to the conditions in which they grew up), warp-enhanced reflexes (I4) but without sense of leadership having an Emperor to watch over them endows Space Marines and Imperial Guard with (Ld5). Their leadership is instead based on safety in numbers, as well as strong pack leader types (myrmidons and grey seers). To survive in the harsh conditions of the warp storms and to advance despite their short life spans, cults were formed based around different aspects of their lives which would ensure that any knowledge they gathered was preserved and worked upon in the future.

The cults I already had in mind are something across between the clans of WHFB Skaven and the Aspect Warrior shrines of 40k Eldar. Off the top of my head I can think of the bio-warfare cult (biotoxvermin and their fanatics), infernal technology cult (warlock engineers, heavy weapon teams, doom wheels, warpvermin), bio-engineering cult (rat swarms and banes), warrior cult (warvermin, warmaster) and assassins cult (shadowvermin and assassins). Ruling above these is the cult of the Horned One (the minor chaos god who watches over the Vermin), consisting of Grey Seers, myrmidons and Verminlord daemon.

Before the psychic build-up of the vermin race was sufficient to awaken the comatose Chaos god the horned one, the planets of the Vermin were content to war amongst themselves using what technology the infernal technology cult were able to construct and maintain, however with the awakening of the Horned One (a quiet, well hidden awakening) he began sending out emissaries, those who would become the Grey Seers. Under the supervision of these blessed (or is it cursed) individuals the Vermin race began to slowly consolidate into a force capable of expanding their borders by conquering adjacent minor systems out of reach of the astronomicon.

This race is not a barbaric race as many 'lost colonies' are, but are not as technologically advanced as the Imperium either. I see Vermin having basic knowledge of technology, thanks to the workings of the Infernal Technology cult passed down from generation to generation. Weapons lack the technology of the plasma or melta weaponry and are limited to mainly projectile weapons, but are often augmented by Warpstone. Their main method of interplanetary movement is hi-jacked trading vessels and Space Hulks, which are predicted and summoned by the Grey Seers, and then made habitable by the Warlock Engineers of the Infernal Technology cult.