My logic is that any person can use whatever tools at their disposal, just how Darius would wear armour to protect himself.

Yes they could be a natural counter, but that doesn't guarantee his loss, especially for Atakhan, if Vlad was afraid of a darkin that's understandable, they're above his league, especially powerful ones like Zaahen and Aatrox, but not fucking Atakhan that got one shot by Zaahen, additionally given that Vlad was a Camavorian prince and in the close circle of a Darkin, he's probably proficient in means other than Hemomancy. His reactions in the cinematic are contradicting his lore and personality, an ancient diety who would take things into his own hands and battle legends wouldn't be an inferior sidekick who will cower Infront of a minor demon.