As I mentioned in another thread here recently, those fins are not VGs. Toyota first used them on the 3rd gen Prius in 2010, and stated in the press materials that they improve directional stability--in other words, they are straightening vanes, similar to but not as pronounced as those seen on hypercars. Here's the 2015 Nissan LMP1 Le Mans car:
Toyota uses them on the current Prius, Camry, Corolla, Sienna, and Avalon.
Here are the diffuser vanes on the current Nissan Leaf, larger than Toyota's but with the same function:
VGs are smaller, since they don't need to protrude beyond the boundary layer, and shorter, since they aren't directing airflow but disrupting it slightly.