I think that's what the previous poster was describing to a T.
We did the same during a recent trip, leaving DD2's RSTV installed as much as possible whenever we parked the rental car, top tethered with the vehicle seatbelt routed as much as possible through the RSTV's shoulder belt and lap belt guides, and only unthreading the vehicle belt from the guide on the vehicle seatbelt buckle stalk's side when exiting the car. You're not required to use the top tether with a lap-shoulder belt but I preferred to do so and it also helped keep the vest in place unoccupied in the car. Then DD2 would just have to put her arms in, buckle up the front of the vest, re-buckle the vehicle seatbelt, route the vehicle seatbelt through the guide closest to the seatbelt buckle stalk, and double check that the belt was routed properly through the other belt guides and snug before heading out on the road again. Not any longer time wise IME than using a "traditional" 5-point harnessed carseat. :thumbsup:
Another tip for hot weather/climates: I brought along a towel to drape over the RSTV and vehicle seatbelt whenever we parked to keep the vest's metal guides and all buckles cooler to the touch when parked in the blazing sun and summer heat.