Yes! This mode of use is tested and approved by Safe Traffic System, Inc. for the vests. Using the tether in addition to the lap and shoulder belt would reduce the rear passengers’ tendency for the unrestrained shoulder to rotate toward the front seat during a crash. Using the tether also offers additional upper body support for special needs children or extra wiggly children.
When using the tether and the lap-shoulder belt, because the seat belt is doing the vast majority of the upper body restraining and not the tether, they can continue to use the single tether all the way up to the 80 pounds. The dual tether is only required when the vest is doing all the restraining in a lap-only belt situation.