Something I thought was interesting about the vampire king was that he had bird feet, not terribly unlike the bird feet that Evergreen had (s6e24). I might be reaching here, but I thought this might indicate some connection between the two?
Evergreen had bird feet because his design was based on a horned owl
And after his vampire essence is extracted, the Vampire King is a lion
No scaly hands, no bird feet. It’s possible that those elements were present in his Vampire King design to lend themselves to his powers — he can fly and he can vanish [like a chameleon], among other things
Some of the other vampires share reptilian features — The Fool resembles a snake enough to be described as a ‘baby snake’ by Jake, and The Hierophant shifts his arm into a snake
And I think it’s pretty fair to say that snakes are a common design element to Adventure Time’s vampires in general
But I can’t really think of any other vampires sporting bird design elements. Not even The Fool, whose unique power is flight.
Bird feet could be a symbolic design element, calling back to Evergreen, to demonstrate power. Maybe they just thought it looked cool. Either way, we don’t expect there to be a tangible physical relation between Vampire King and Evergreen. Vampire King is a lion.