my understanding of why Vriska treated Tavros like that in a broad sense has always been that she could see the potential he had and wanted to help him reach it, but she didn't realize that his way of growing and becoming a fully fledged Big Boy Tavros was going to be inherently different from her own path (because Tavros isn't transfem like her) (he's a chaser /j).

and so eventually she gave up on him and moved on to June, and from then on everything with Tavros is just being frustrated that he ends up finding his own, entirely different way to reach his potential. this, of course, ultimately leading to the great ghost migration of 2016