Kendralind (the Sithicus kender village) was brought into Sithicus from Krynn by Lord Soth probably with some ritual and then he experimented on them as you described.Drinnik Shoehorn wrote:Kender vampires only exist in Sithicus and are the result of Soth experimenting on a small village of kender that appeared in the domain when it was formed.
From the Ravenloft Monstrous Compedium 2e
Knowing the revulsion that the elves who live in his domain feel for all manner of unnatural things, Soth felt that he could find no better slaves than a band of undead. Aware that the undead elves might pose a threat to his own power, Soth set about the creation of a new breed of undead. Drawing a small kender village through the misty veils of Ravenloft and into his domain, he had them killed one by one so that he could study their suffering and invoke carefully designed rituals over their bodies in attempts to make them rise as undead.