Tavros is incapable of feeling hate and Vriska is incapable of feeling pity. They're the perfect couple!
I don't think she suddenly became a nice person. Sure, she gave Tavros the code for her rocket shoes, so he could alchemize a rocket chair, but that isn't quite enough for me to move her out of the HUGE BITCH BLUH BLUH category. Sacrificing the five minutes it takes for her to get the boots and check the code when Kanaya ordered her to do that and threatened her with a toilet. Especially since Tavros only needed the rocket chair because he can't climb the stairs
Vriska built because
Vriska tossed him off a cliff.
But I kind of find their relationship too alien to relate to properly, and just think it's hilarious. Vriska hates Tavros, and this is a normal troll emotion, and Tavros is the weirdo for not hating her back.
... Wait, are John and Jade actually dead?
*SIGH* Look, it's perfectly simple. Terezi talked John into sequence breaking and fighting his Denizen when he had no chance of beating it. John predictably died, and with no one to be her server player, Jade was crushed by a meteor. Dave and Rose played ahead for a couple of months, trying to get as far in the game as possible. At some point, Dave created the Time Tables, which he used to travel back in time and warn John. He then gave all his future loot to Past Dave and prototyped his kernelsprite with himself. By the time Dave traveled back in time, both him and Rose had already awoken on Derse, and Future Dream Rose survived when the timeline where John was dead ceased to exist. Future Dream Rose becoming Past Dream Rose when the timeline collapsed woke Rose up, and she could fly over to Dave's tower, where they had a dance party.
Thus, both John and Jade are alive in the current time line, which is in part defined by John and Jade dying and Dave traveling backwards in time to stop it.
And Terezi is the Best Troll.