Vriska isn't a Mary Sue - mostly because none of the other character's in the strip like here aside from the ones that don't know her. In fact, most of the character's seem to treat her like a necessary evil. I doubt most would care if she was killed.
Sollux, despite his somewhat depressing nature, actually seems well liked, AND he gets the girl, all while being super powered! He represents both what so many geeks want to be in addition to being enough like them to be sympathetic.
Vriska is spending a lot of time building herself up, making herself look big... but the bigger they are... and she's. just. so. stupid.
She doesn't plan. She doesn't think ahead at all. And I really believe that she has absolutely no idea what she's getting herself into.
Her success thus far is down almost solely to her endless if pointless ambition and a surplus of extreme luck - and there are plenty of real life, realistic character who got into amazingly important positions in the same exact way. And in the process she has alienated anyone that would care about her enough to help her should that luck ever run out.
Maybe it's just me, but that doesn't say "Mary Sue" to me. It COULD become a Mary Sue, but it isn't there yet (and I don't think it will turn in that direction)
Also, I just totally discovered that konquerer allows you to have two... windows?... I don't even know what to call it anymore. Two somethings open in the same tab, side by side. Crazy.