vriska doesn't have an ounce of cunning in her. she repeats the same mistakes over and over, and she assumes that everyone thinks the same way she does. like when she doesn't get why people are still mad at her even after getting their revenge, for example: she thinks an eye for an eye (or two) cancels things out. obviously not everyone agrees.

she does have a lot of brash bull-headed courage and conviction that she's doing the right thing, though, which is what makes her she read as gryff to me.

slytherin would be my second choice. she does have a lot of ambition and likes to influence people (even though she's garbage at it without her powers). it's just that she acts on these traits by being rash and throwing herself and others into danger in ways she thinks are heroic and right.