It sounds like your dealer may have sent his B team. Call and talk to the owner, ask for the A team, or ask the owner or manager to come out and supervise this time. If your tub is suspended by just the corners, filled and left for a while, you can have a problem that will end up in a finger-pointing battle.
If you shim the spa and get most, but not all of the spots, you'll be fine. I have seen HotSpring tubs go years and years with the corners and three, four or five spots in the center supported with no damage, but there is absolutely no reason to test this!
There is another possibility: if the delivery team saw that the center of the tub is in contact with the slab but the slab falls away at the corners, then they did ok. You mention a half-inch slope. That's not bad if the tub is well supported, but if that's to much for you, then have them come out and level. And again, if the tub is held up by just the corners with air in the middle, don't fill till you get it right.
If you want to do it yourself, allow several hours for a 'first timer.' There are about 16 spots which need to be supported on the bottom of a Vanguard. Look at the "Pre-Delivery Instructions" which your dealer should have given you. If not, go to Hotspring.com and download a copy.
I use a special 'Spa Jack' from the same people who make the 'Spa Dolly' and it works like a charm.
If your dealer doesn't have one, you can simply lift the spa
from the corners with a 2x4 and a some blocks. One block goes on top of the 2x so you don't lift from the siding. This block can be an inch thick or so - if I have to do this I use a 3/4" thick stake, and if I can, I nail or screw it to the end of the 2x so it stays in place. Look at how the spa jack, pictured above, lifts the tub without hittinig the siding to understand this extra little block.
Then of course you need another block for the fulcrum, and that can be of various thicknesses according to what you need. I only lift the tub about two inches at a time, then put in a block to hold the tub. Then I move the lever over to the black part of the bottom and lift again until I have the tub as high as I need.
I usually start by putting the level down on the slab to get an idea where the high and low sides are. If you have a nice even slope in one direction, it is best to try to end up with one side of the spa in contact with the slab.
Raise the low side, and put in a temporary block to just hold it. You should try to end up with the spa sitting level on two or three blocks. The delivery crew may have done that for you already and just not gone on to the next step: shimming the spa.
Once the spa it level, you need to slide in blocks to fill the gap between the leveling spots and the ground. You can use redwood blocks cut to the size you need, but plastic or pressure-treated wood is better. If you are very lucky, you may end up with a nice even gap. What I mean is a gap that is the same thickness from one side of the tub to the other. In that case you can measure the gap and create a single long shim that simply slides into place from one side of the spa to the other. But most likely you'll need blocks or stack of shims to support the spa. Sometimes you can put them in a row in the right places, and lower the spa onto them in one move. Saves lots of time, but rarely happens.
I use plastic shims - Called "EZ Shims:
and some cedar blocks. If one shim isn't thick enough to close the gap simply stack them. If you have the time and patience, put two stacks side-by-side at each point for extra support.
If you have a gpa in one or more places that is really thick, and it doesn't sound like you do, you can add blocks. I buy cedar stakes and cut them up into blocks. Cedar is good, but if you can put the shim under the block to keep it off the slab it's better.
Closer to the slab is better. Don't end up with your spa sitting on top of four-inch stacks of blocks unless there is some special reason.
