Sorry, just to confuse matters a bit more - the decision about using a top tether over the vehicle's tether anchor weight limits does NOT apply to the 86Y harness.
This is because the top tether on a carseat is a supplemental
restraint for the carseat - it helps the primary restraint (lower anchor strap or seat belt) work better.
With a harness like the 86Y, the top tether is a primary
restraint for the harness, therefore they cannot
be used over the weight limits - no tough choice/parental decision there.
I've never tried the 86Y with a Vivo, so be aware that it's not compatible with all HBBs. If the "neck" on the Vivo is too wide, it will displace the shoulder straps on the 86Y.