I agree with Mare about the 1st person thing. If you think it will be easier to write and be a better fit for the story, switch it... but definitely put an author's note, and eventually, you should edit the beginning to be 1st person too and re-post those chapters. But like Mare said, you don't need to do it right away as long as you explain the situation.
Vampires ARE a popular subject, thanks to Twilight, but there's also a lot of backlash about vampires now, too. That's the case with anything that gets so popular. You will have people who won't want to read a vampire story solely because they're sick of hearing about vampires from Twilight mania and all the vampire shows on TV now. But vampire stories have always been popular in the Bsb fandom. Paranormal stuff usually does well, too.
Romance (whether with drama or comedy) definitely seems to be the most popular genre of fanfic, especially on AC, so you find that easy to write, go for it - you will probably get more readers that way than with a vampire or even a paranormal story. And like Mare said, if you use AJ or Nick in the lead, even better. AJ romance and Nick romance almost always get more readers than stories about the other guys or non-romances. But only go that route if that's what you really want to write.
I completely understand your frustration; I do think we all go through this kind of "writer's crisis" from time to time. My most recent one was back in June, when I kept flitting from story to story and started two new ones. I hate having a bunch of unfinished projects in the works at the same time, but sometimes you need to just experiment and start new things until you find the story idea that's going to work for you. You don't have to post everything you start, but don't be afraid to give these different ideas of yours a try.