The point is not that we have 1 unit (a stack, in other words), but that after applying "Vampirism" to the Kshatriyas, they become undead with all the consequences that follow, and it will be impossible to control them without Sandro's cloak (this cloak is possessed by Biara in the last mission, so you need to be careful with her).
I also had Zehir gain Vampirism after replacing Luck with Darkness in the first game, and I tested it specifically on Rakshasas – it's very effective.:smile54: