So this is a little strange to say, but I'm not actually sure what to translate the 「えっ」 as. What does that mean in this context? Is she taking it as self-evident that she will be a vampire some day, and it's an affirmative, "Yeah[, I know. I won't be human though]"; or is it a truncated interrogative, "Really?" That the statements are identical and share a similar locality, I'm actually leaning toward the former, but the doubt is sufficient that I want to be sure.

DschingisKhan said:
or is it a truncated interrogative, "Really?"

This. The っ indicates a glottal stop (or, at least, a sudden pause) in this context, so they're meant to sound surprised.

Also, an affirmative would usually be ええ (or えぇ, or えー) rather than just え.

I'd say something like
"Yeah?"
"Yeah."

or
"Really?"
"Really."

Even with the glottal stop truncation, one can inflect えっ as a statement rather than a question.