Well, honestly, it would be easier to force character interactions if the 16 year olds-24 year olds had to train in groups. Other than that, a nice plot opener would be someone being forced out for being a vamp. They then start noming on people and that could be motivation enough to start some of those born human to want to try to stop them. It would be a gradual thing.... It would also cause friction between both worlds and how they act. Actually... I'm assuming vampires rarely happen right? Maybe once every 400 or so years you get a dragonkind stupid enough not to make the decision? Or, even better, it's not as bad as it looks and as people make out to be... You said they suck human blood, not that they had to drain it... Just an idea. I prefer my baddies in the style of GRR Martin. Oh these whitewalkers... oh you think they're evil... I'm just gonna dissuade that in my interview here... K thanks. (I'm not sure if you understand what I mean, but if not, you really need to go read A Song of Ice and Fire....)