I think the vest you're talking about is the Ride Safer Travel Vest? Keep in mind that it also requires a tether when used with a lap-only seat belt.
As for the 86Y, the tether anchor that came with your van has a weight limit, so for kids using the 86Y over that weight limit, you have to install a heavy-duty tether anchor instead. That job can range from pretty simple in a sedan, to impossible in other vehicles like vans and SUVs.
Now, as for the weight limit: in this case you have a Mazda, and since Mazda defers to the carseat manufacturer, that means you go by the limit set by EZ-On. They have an 80lb maximum-allowed weight for anything except their heavy-duty tether anchor. If your 9yo is over 80lbs, you'll have to install the heavy-duty tether anchor to use the 86Y. Also, if your 9yo is under 65lbs, you MUST use a booster with the 86Y - it's required.
I have an 86Y and I like it for what it is, but I think you'll find that it's even more of a hassle than just using the Radian in that seating position.
