Singularity Sourcebook, "Naalir," page 18 - "The [sublight alien ship discovered in -4275] was large by Vilani standards, approximately
750,000 tons. Its engines were strange, not at all like those that Humaniti had developed, nor those of the Vargr, Aslan, or any of the other species the Vilani had encountered."
By -4275 the Vilani Imperium had contacted neither the Vargr (first contact -2700) nor the Aslan, and the latter were probably never contacted during the First Imperium. The expanding Vilani had contacted numerous human and non-human races by this date, but very few -- maybe only the Geonee (first contact -9000) -- had jump drives that weren't directly or indirectly descended from Vilani designs. Some Droyne communities had rediscovered jump drive around -7000 but it's not clear if the Vilani had contacted any of them by -4275, and even if they had First Imperium Vilani didn't realize the disparate Droyne communities all belonged to a single species.
Maybe the easiest edit is just "Its engines were strange, not at all like those of any of the other species the Vilani had encountered."