A vampire most certainly can get access to it if they are clever. To get the schematics you have to be a member of the Dawnguard as Sorine Jurard is the only one who can smith them and teach you to do the same. All a vampire has to do is (go undercover) seek a cure and join the Dawnguard after they are cured. Since you already killed the Dawnguard, your vampire can never get access to this weapon.