The best answer for this is, “it depends.” As CPS techs we would find out more about your situation and recommend the best solution for keeping your child safe which may or may not include the RideSafer vest.
The XL vest is certified to 110lbs. That certification is based on the lap-only belt and tether configuration.
If someone is using the RideSafer and the vehicle’s lap-shoulder belt the RideSafer will continue to do what it is designed to do and properly pre-crash position the seat belt. Even though they would technically be outside manufacturer specs once they understand the design limits they have a parental choice to make.
Sometimes another product is the best solution. If a restraint bigger than the XL  extra is required, we would recommend a Merritt Manufacturing Churchill restraint.