Was the concept of "vampires die if they fall in love" actually true? I've forgotten at this point, if so that could be what killed them if not the sun. That's obviously also an obstacle to the MC and his vampire boo too.
Overall this arc left a sour taste in my mouth. We basically just watched a bunch of people try to stop and then eventually just give in to allow a high school kid and his MUCH older gf to go through with a suicide pact. I didn't even think the guy's life was in danger until the moment it showed him dustified, but it seems the main characters just accepted that was a possibility and wistfully saw him off to go kill himself. They don't even seem that surprised, like wtf. This just came off as gross and depressing rather than a hopeless romance.