I have to say I agree with the idea that Dracula is most vulnerable to his own power. Bulgarian folklore does state that a dhampirs were specially selected as vampire hunters since they possessed the vampire's advantages, but none of their disadvantages.
Now, let's not forget alchemy became a very important part of the story after LoI. I strongly believe Dracula in the end IS weak to alchemy. Why? The VK was an alchemical whip before being infused with Sara's soul, and her soul was trapped in it using alchemy. I'm not expert in the subject, but it sounds to me that the power of the whip and Dracula's power (which comes from the Crimson Stone, made by alchemy) resonate in some sort of alchemical process. As far as I know, alchemy repeats over and over again the same process of burning, distilling, grinding, etc. to purify the materials. Maybe Dracula's soul was purified by being subjected to the same process of dying and resurrecting over and over again, because alchemy was applied to it over and over again.
On the VK: this is complete headcanon, but since the Belmonts have witch blood thanks to Sypha, even if the whip itself was completely replaced, Sara's soul could be infused in the new whip by mystic magical means. Juste, for example, can fuse spells with weapons. So, only in theory, they could infuse the VK's spirit into a new vessel (which might or might not have been taken as a sign of disrespect by the lady living in the old whip and might or might not be related to the bloodline being unable to touch the whip till 1999 and stuff). Then again, this is pure speculation so, yeah, whatevs.