[Mike Wightman]

The Vilani only became 'conservative' in the run up to the consolidation wars and the Ziru Sirka period that followed. They had a considerable drive for exploration and development prior to this.

Even during those eras there would have been malcontents, Vilani who were told tales of the old days, hybrids, minor human races, 'aliens' would all contribute to this trickle of exiles, refugees and migrants from the Ziru Sirka.

Once the collapse of authority began, which was happening long before the encounter with the Terrans, this trickle would grow.

During the later ISW period you had Ziru Sirka worlds defecting to the Terran side, that tells you a lot about the stability of the cultures of those worlds. Despite a thousand years of tradition they preferred to throw their lot in with the Terrans.
This news would spread and the trickle is now a small stream.

The final Terran victory and the appointment of a Terran emperor, the new drive of cultural reformation, the incompatibility of Terran and Ziru Sirka ways of doing things and you now have the potential for a river of exiles, migrants and refugees, setting off into the wider galaxy to seek their fortunes much as their very distant ancestors did.

The smarter ones will have raided as much of the secret knowledge repositories as they could before heading out on their new adventure.

During the long night pocket empires and high TL worlds still had the potential to launch exploration, scouting and even colonisation missions, its just there was no longer a central 'Imperial' authority laying down the rules.

The Syleans didn't expand their federation into a power vacuum, they expanded into a region of space where there were many cultures, worlds and pocket empires. They invented the fiction of the long night to suggest their authority and to belittle the lesser pocket empires they encountered.
They made them an offer they couldn't refuse - until they ran into the Jullians...