Vidal would necessitate a significant shift in playing style. Vidal cannot control, distribute and dictate the game like a CM needs to for Barcelona. Vidal for Barcelona would either be a CDM in Busi's spot, effectively putting him in a position that would not allow him to showcase all his talents, or he would be a box-to-box midfielder in a double pivot system and Barcelona would then switch to play like Man City plays now or Bayern used to play under Heynckes.
Kroos on the other hand, would not require a change of system, just merely an adaptation period. He can slot into the CM role, and while he won't necessarily play like Xavi, he has the skills to still distribute and dictate the game. He is a much better fit.
Kroos will almost certainly cost a hell of a lot less.