A promising start
This story has the makings of a good one. There isn't much logistics one would expect with vampire stories, and sure, while there are a few concerns over how the taking of slaves would be considered by the human community and such. The story has a good romantic line, and one would probably find themselves patronizing with the hero rather than both the protagonists. Being kings, I would find it more appealing to have more political subplots that necessitate their actions, not just ordering other characters around to call them by their royal titles.
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Overall, a well-conceived plot which deserves more note as to the implications in their actions. I would suggest the author to try expanding her base to include more than just romance since her writing skills are good.