Just because his name is Vladimir doesn't mean he's a vampire, Faoline. He's been magically infused with the blood of every hemomancer before him. He's a human, and a mage, but not a vampire.
While they could write it in, that could be said of anything, including, say, the idea that Vladimir is a time-shifted sentient penguin god who has chosen to take the form of a mortal human in order to be able to get closer to Vayne, the reincarnation of his long-lost love. Just because you can do something doesn't mean you should, or that it's a good idea.
Another way of looking at it is this: Trundle's rework changed the character to fit the world, but kept the kit constant. Sion's rework changed the kit and the character, but kept the same space in the lore and simply fleshed out and expanded the character to fit his original intent better.
Meanwhile, your suggestion creates an entire new side to Vladimir that drastically changes his kit and lore, without really adding anything to him either in or out of game. Vladimir's lore is actually pretty decent (although it could use a bit of revision to tie it into the rest of the world more), and the major issue with his kit is that free healing is exceedingly powerful: even now late-game Vladimir is a terrifying monster...he's just not reliable enough on his way there.
I'd be much more interested in a lore rework that explores who Vladimir is and how he fits into the world (rather than adding things to him in a slap-dash attempt to make his kit work), and much more interested in a kit rework that looks at his current kit, identifies the problems that are causing him to be intentionally kept weak, and proposes fixes for those issues while retaining his feel as a blood mage. This suggestion addresses neither issue, and, in fact, raises even more issues with regards to how his lore and kit would work.
Further, current Vlad players enjoy Vlad for what he is: a blood-draining ranged harass mage with a strong late-game, good durability, and an excellent sustain once he gets past his shaky early/mid game. Your suggestion necessitates not only a weakening of those aspects (as if he was current Vladimir and had another kit with its own strengths he'd be overpowered), but creates an entire separate side of the character that, it seems, is strictly at odds with the current kit. All current stance champions encourage mixing and matching the stances to get the most out of a kit...but nothing in Vlad's current kit is particularly out of place, so he has no real incentive to switch...nor any WAY to switch, unless you're cutting an ability from him.
That said, the best way to convince people like us is to show us a kit. Show us how you'd create a stance-swapping Vladimir that still feels like Vladimir, and show us how you'd tweak his lore to make it seems like a single story and not a lot of tacked-on elements to justify an end result. That sort of effort merits a lot more attention and critique than "here's a suggestion" followed by no ideas on implementation.
