I'll start saving them up again :)
Of course, it's easier to tell when nobody knows a book if it's the only one and no replies.1.
My Sister, The Vampire A teen or pre-teen boy, his slightly older sister, and their much younger (seven or eight?) sibling are out in their family's cabin for vacation. Their parents get called away, but say they'll send over the great-aunt to watch after them so it'll only be for a day or so that they're alone. For whatever reason, the great-aunt doesn't show up on time, but as she's a big pain in the butt the kids are THRILLED and are doing a great job of taking care of themselves. (For kids who've never so much as made pasta before, that is.) They're making friends with the family down the way, which has two siblings that share a room (maybe twins?)
There's vampires. They're vaguely creepy and are determinedly interested in the various kids because (we find out later) they want a family of their own. So when they're not being creepy they're trying to win these kids over with, like, sports pep talks and all. The kids get vaguely weaker and have "weird dreams" which end up with the windows open, but they wise up and start putting up crosses and garlic and so far so good... when the sensible pain-in-the-butt great-aunt shows up! And takes it all down! And goes into their rooms at night opening windows for their "health"! And the younger sister is wasting away and positively has to be bullied to wear garlic *already*...
The vampires end up staked to the ground as the sun comes up, resigned to their fate, and the kids feel vaguely bad about it all. (But triumphant.)
It's a late-kids or early-YA book, I read it probably in the early 90s. Cover had a picture of the family on it with the teenaged girl dressed in a "goth" style, but not really.
2. I remembered this one today. A bunch of kids is out camping as a school group or something during Christmas break. They're from London, and camping in England. On the trip back at the end, they do notice that either there's no traffic or that all the traffic is coming away from the city but not toward, but they don't think too much of it. Either there's no radio, or there is radio but it's all canned broadcasts, and they think this is a little odd, but again, they're not too worried.
They get to the city and it's all barricaded. At this point, for some reason, some or all of the kids (teens) get separated from the grown-ups. One of the kids tries to get into the city through a barricaded house, a pair of kids is there and one of the kids says he went in through the broken window, his sister freaks out. Their mother appears and, thinking the teen came out from the city, declares that if she had a gun, she'd shoot her. The teen is freaked out and disappears into the city.
At this point the book diverges and we get some chapters from her(?) viewpoint and some from the viewpoint of a (Muslim?) boy who was in the city while our intrepid heroes were out camping. (They eventually meet up.) From his perspective we hear that there was a bad plague... maybe vaguely ebola-like (which would put this YA book firmly in the mid-late 90s), and the city was quarantined. He may have a younger sister who may or may not have succumbed. His mother is definitely dead, and he's using his stash of condoms (that he never used for the intended purpose, I believe) as a way to touch things that might be infected so he doesn't get sick. The public service announcements, when he gets radio, are on a loop - things like "don't use public transport". He thinks this is a laugh, because the only "transport" left is your own two feet. All service to the city has stopped.
At some point after the kids meet up, they realize that there have already been people in and out of the city, and there may have been a few news reports of the disease spreading (the kids hypothesize, I think, that it may have been biological warfare or "friendly fire" from their own government developing a supergerm but letting it get loose accidentally), so - since there's pretty much no food and very little left in London - they might as well sneak out and leave anyway. And they do... and I think it may be worser deserted out there than it was in the beginning of the book. Dismal book, really, I don't know why I want to find it again.
Edit:
Probably Plague, by Jean Ure.