We don't consider balance here because a mode with both Vampire and Cursed Soul is absolutely not made with balance in mind anyways. Difficulty to code could become an issue though.
The Vampire role will always start on the Vampire faction, but can be swapped by CS.
Once the Vampire role dies, regardless of what faction it was on, a new Vampire role will be generated in the Vampire faction using the existing mechanics.
Any faction with a night chat (Coven, Apocalypse, Cursed Souls, Pandora, Compliance) will be able to convert other Town members to their faction.
Any faction without a night chat (besides Town) will be able to convert other Town members to their faction. The converted player is revealed to the Vampire on a successful convert, but the converted player doesn't learn the Vampire's identity.
Town aligned Vampires will always deal a Basic Attack to their target instead of converting.