"Grounding" any electrical device means that you have a wire going back to the power source and connecting to the ground bus or ground connector in that electrical box. So - a 'ground' wire runs from the spa, through the sub panel, and back to the main panel. In your Vista, it connects to the ground lug inside the spa, the ground bus inside the sub panel, and then there is a wire of the same size going from the sub panel back to the main panel, where it attaches to the ground bus or lug.
"Bonding" is a complete separate system which connects all metal objects in the vicinity together. This usually means ALL - and includes things like windows with metal frames, patio covers (metal ones only, of course) posts or pillars, any metal object near the spa. You need to check with your local building and safety office to find out if you need to bond the slab, but it is real easy - put a #8 bare copper wire onto whatever metal you put into the concrete, and leave enough of it sticking out to reach the bonding bus on your Vista. Some people buy a 6' copper ground rod and lay it in the concrete with the end sticking out. Then they use a ground clamp and copper wire to go to the spa - and any other items the inspector wants you to bond. The idea is that if ANY electricity gets into these metal items, not necessarily from the spa, but from ANY source, it has a path to ground which will not include YOU.
BTW - the Vista has a Ground connector INSIDE the control box for that purpose, and a Bonding Bus on the OUTSIDE of the box. Some electricians mistake the two.
HTH
