Putting vortex generators at the trailing edge of a surface, as shown in the above photos, is pointless.
To be functional they need to be in a region where the flow has not already detached, and their purpose is to enhance attachment to the downstream surface. In the photos, they are probably in a region of detached flow and there is essentially no downstream surface. Wrong on both accounts.
They may not reattach the airflow at the end of the vehicle, but perhaps numerous small vortexes have less drag than one big one. I could see them working like the trailer tails on semi trucks.
Having been involved in transportation and logistics for over 30 years I'll provide a simple observation. If you don't see the likes of Schneider, JB Hunt, UPS, FedEx, Ryder... adopting it onto their fleet vehicles there is a reason. I've been part of many a study on how to make tractor/trailer combos more efficient. After payroll, fuel is the number one expense. Anything you can do to cut it even by single digit percentages is adopted immediately by these organizations.
A simple cost benefit analysis done by these organizations will tell them if it is worth it or not. These guys aren't looking to go faster, they are looking for more fuel efficient. Coincidentally, MPG vs MPH have a common thread - aerodynamics. Trust me, these large carriers have annual fuel spends in the hundreds of millions of dollars. If they could save 1% with the bolt-on devices they would outfit their fleets overnight.
Always look at the big carriers and what they adopt, the little guys or the one-off guys are easily swayed...I've seen it a hundred times.