I'm gonna start this off by saying that my knowledge of genetics and blood types and such are a laymen's knowledge at best. I didn't even take high school bio.
Also, this is a mixed fantasy/sci-fi setting, so I'm trying to account for both those ideas.
From a purely sci-fi point of view, I'm pretty sure different races would not be able to share blood through transfusions - they would simply be incompatible. However, some species are able to interbreed and produce viable offspring. (Nobody has ever said that half-elves and half-orcs are sterile.) To me this means that, at minimum, orcs and humans, and elves and humans should be able to share blood, since they share enough genetic similarities to produce offspring.
And from a fantasy point of view, polymorphing magic can make a dragon into a human to produce a child. So magic must change genetic structures and gamite cells enough to once again produce offspring. To me this means that polymorphing magic could change one's blood type, for example.
Vai's blood is neither that of a half-elf nor a dragon, but some twisted middle-grou d wrought by dark science and magic.
This is where my conundrum comes from.
I'm honestly fine either way with this problem being resolved quickly. I'm just trying to be as medically realistic as possible.