What I mean by will is that those under the power of ex LK Arthas are not entirely mindless, no rational thought, does not have their own preference nor have any trace of choice. They got new LK now maybe he's not strict as compared to the later guy.
Sylvanas has her own will but was only able to exercise it when eLK's power weakened and his entire control of those undead.
The vampyr race if implemented can use the withered existing legion model as their beast/batform visage. NE or elves have batlike similarities think banshee or male equivalent. They too have cast and attack animation and can hold weapon.
https://m.youtube.com/watch?v=_i933nMLZKI
A likely subject for vam'pyr strain and sub group would be:
Blood Elves loyal to Kael'thas
Nightfallen of Azsuna and Suramas as well as withered
Existing San'layn not yet risen or who are hiding.
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The reason why the known San'layn attacked the Horde itself is for a social experiment reason. Don't you think Sylvanas wasn't aware of it nor she has not given approval nor the signal to do such?
They are trying to increase their population and most probably finding the perfect subjects for them to maximize their power as it seems Vam'pyr trait would be most compatible to a specific race.
They're nothing more but like Void Elves although they are turning others unwillingly but hey, aren't the worgens or the Forsaken not any different by those who were bitten? It wasn't their choice but it seems that the hand of fate chose them to be who are they now.
Apart from using Val'kyrs to raise those who have fallen and include them as a part of the Horde as Forsaken, if one choose to do so, the San'layn "infection" would be another way to bolster their numbers and ranks as some San'layn might experience regaining their free will when the old Lich King died and freed them from slavery or bondage to do his bidding.