Although the background suggests Vostroyans do not ordinarily use Roughriders, I could not resist the modelling project and so created a story to justify having them in my army:

Horses are completely unheard of on the snowy, industrialised wasteland of Vostroya. So it is no surprise that Imperial Guard Regiments raised on Vostroya do not contain units of cavalry. Yet in an organisation as vast as the Imperial Guard, strange examples do exist of the firstborn sons of Vostroya taking to mounts. The tale of the Vostroyan 22nd and their campaign against the Orks of the Kurkaris system is one such case.

By the time the Vostroyan’s arrived on the ice blasted surface of Kurkaris the remaining Orks had already been driven into the dark depths of the systems equatorial forests. The dense terrain meant armoured units were unable to enter and with the last of the Orks fighting a bitter hit and run war, a solution was needed to cleanse the last of the Greenskins from Kurkaris forever. The task was set for the makeshift Hussar squadrons of the Vostroyan 22nd. The Hussars were formed with the help of local PDF militia cavalry to ride down and cleanse the last of the Orks. Drawn from the tank crews who now swapped their armoured steeds for ones of flesh and blood they swept into the forests with characteristic zeal to see the task done. With hunting lances and flamers they stalked and cleansed the scattered bands of Orks.

The campaign was successful but costly, a cornered enemy fighting to the bitter end. With less than a quarter of Vostroyans who entered the forests returning the remaining Hussars had formed a unique bond. In honour of the sacrifice made by the armoured units the remaining men were allowed to retain their horses, but only after training their newly arrived replacement tank crews from Vostroya. Originally formed as a temporary unit, the Hussar squadron of the Vostroyan 22nd continues to fight on in the name of the Emperor.