They definitely tested it. Here's a paper from Toyota.
http://www-nrd.nhtsa.dot.gov/pdf/esv/esv18/cd/files/18ESV-000541.pdf
It's also been independently studied.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3217442/
And my insurance company charges slightly less for vehicles with VSC from what I've noticed.
http://www-nrd.nhtsa.dot.gov/pdf/esv/esv18/cd/files/18ESV-000541.pdf
1. It is estimated that the accident rate of the vehicles with VSC showed approximately a 35% reduction for single car accidents, a 30% reduction for head-on collisions with other automobiles, and approximately 50% and 40% reductions for accidents where severe or moderate damage occurred, compared to that of the vehicles
without VSC.
2. It is estimated that the casualty rate of vehicles with VSC showed approximately a 35%
reduction for both single car accidents and headon collisions with other automobiles.
It's also been independently studied.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3217442/
Overall, and 11.2% (95% CI: 2.4%, 21.1%) reduction in multi-vehicle frontal crash involvement was identified for VSC-equipped vehicles. A 52.6% (95% CI: 42.5%, 62.7%) reduction in single-vehicle crash rates was found.
And my insurance company charges slightly less for vehicles with VSC from what I've noticed.