Vivian is the maligned DiLaurentis child who, for reasons of mental illness, was shuttered off to Radley as a kid. There she befriended Bethany Young and eventually broke out of the nuthouse. Jessica DiLaurentis then cozied up to Bethany to try to find out what her deranged daughter was plotting.
But Vivian is off meeting Cyrus, in the "Strangers on a Train" situation. If you haven't seen it, you should rent it. It's my all-time favorite Hitchcock movie. But the gist is that two strangers meet on a train and half-joke, half-not-joke about doing away with each other's "problems," i.e. an overbearing father and a cheating wife. One of the men doesn't realize how serious the other man was taking things until his wife ends up dead and the man from the train wants his favor repaid.
Anyway, in this situation, Vivian and Cyrus make that kind of agreement -- she's going to do something for him and in exchange, he'll kill her sister Alison. Except instead of Vivian being the one who didn't take things seriously enough, her plan is to never go through with her end of the bargain and instead, just have Cyrus dispose of Alison and then hold that over him so that he'll do whatever she wants.
But on the night it's all supposed to go down, something goes wrong. I don't know what -- the whole day/night of Alison's disappearance is, honestly, so full of activities that it had to have been like 50 hours long. But let's say Cyrus chickens out and Vivian actually clobbers her sister with a rock, which is why Mrs. DiLaurentis covered it up and buried Alison.
Then Mrs. Grunwald rescues Alison and she escapes. But meanwhile, Bethany Young, thinking she's helping her friend Vivian, shows up dressed as Alison and Cyrus finally gets up the gumption to go through with killing "Alison," so he clobbers Bethany and runs off.
I don't exactly how that all goes down, we don't have enough information yet, but that's what I'm going with. Anyway, Melissa then sees Spencer standing over Bethany's body and buries her alive.
Vivian is "A" and has had Cyrus as one of her helpers all along, because she can hold this secret over his head. It's also why he attacked Alison in that basement (on Vivian's orders) and it's why he confessed to kidnapping Ali.
Or maybe this Alison, the one who has been claiming she was kidnapped and has a scar on her leg, is actually Vivian (the scar is from a real fight between her and Cyrus). And since she's "A," she knows all the Liars' secrets and has had a lot of fun screwing with them and taking over her sister's life. And the real Alison? Dead somewhere after all.
What do you think, "PLL" fans?