The worst part is still the logistics of getting enough blood to sustain oneself.
The vamp dragon needs the blood, the sac holding said blood is useless and actually harmful because eating solid matter of any kind tends to be painful for a vampire. So it can’t just chow down on a city full of humans to get it that way.
Unless the dragon is going through the tedious process of draining cities full of humans by cutting their heads off and squeezing the blood into a giant glass, its just not going to work. (And that wouldn't be efficient anyway)
And that’s assuming the humans are going to just sit around and take it before they start calling in paladins, monks and other holy types to slay the beast. It’s just not worth it all around.
This means ideally the dragon needs to get a steady supply of blood from large creatures.
Hunting other dragons is the obvious solution, but that’s problematic. A normal vampire has the luxury of having a large herd to feed from. Dragons in general don’t tend to hang around each other except to mate maybe and they aren’t very common in general.
I suppose feeding on giants might work, but they’re still not as common as humans. The dragon is probably going to deplete a tribe of them eventually.
There are also all the other weaknesses that normal vampire has that can be applied to the dragon version and I don’t really see it as an advantage for a dragon.
However, I can think of a scenario where it might work out and I”m just making this shit up since this isn’t real life and instead an incredible simulation.
The vampire dragon needs to live in the grim dark north where its going to be night for longer periods of time. That minimizes the biggest sunlight weakness problem. It’s undead so temperature isn’t going to be a problem either.
The vamp dragon is going to have to enslave/dominate the local frost giant tribes much in the same way some normal vampires do to human peasant villages. Giants tend to buckle under anything stronger than they are and probably aren’t going to have holy warrior types among their ranks. With this group of underling it can potentially get sacrifices on a regular basis.
Presumably the giants need to eat as well, so let’s use the old stand by of mammoths. These are pretty big and if the dragon really needed to, it probably could drain a couple of these from a pack once in awhile.
Finally, frost giants have been known to have white dragons as pets, so the vamp dragon might even be able to get some proper dragon blood as well sometimes.
Still even in this ideal situation the vamp dragon is still more or less having to “make due” with inferior food most of the time and not being at full strength. A frost giant or a couple mammoths might keep it fed, but its like when a normal vampire is feeding on rats. It’s mainly doing it out of desperate survival and to keep from going into a vampire coma.