It's a trap that many storytellers fall into: after sending the hero out on a grand adventure, he's got to go on another one, and then another one, each one grander than the last, until finally the series is straining under the weight of a story arc so huge that everyone just wants it to end already.
Rosario+Vampire hasn't gotten that bad yet, but if this volume is any indication, it's heading there fast.
It starts out with an all-too-familiar but mostly innocuous school-comedy cliche: Tsukune has to help out with planning the school festival. It could have stopped right there, rolled out a few slapstick moments, and moved on. But no, the preparations for the festival quickly snowball into the groundwork for the next Big Fat Adventure: there are students on the committee who Tsukune killed, and the monstrel organization wants to take over the entire school, and the main villain of the arc is a dead ringer for Light Yagami of Death Note. Which should give a pretty good indication of how bloated and convoluted this is going get. Deceit, double-crosses, and dramatic revelations are all part of the storyline- which normally would be a good thing, except that every single twist can be seen coming from several pages away.
And that's the essential problem with this arc: it's a suspense thriller where all the thrills and suspense are the result of predictable, textbook examples. Witness the scene where Tsukune has a "Hey, how did you know something about me when I never told you?" moment with a student spy, or when the Death Note guy cackles about his elaborate set-up going exactly according to plan. Even worse is when obvious plot devices are used to force the story in a certain direction: yes, it sure is convenient to weaken Tsukune by having Moka get kidnapped and be rendered useless for a few chapters.