i am no vault expert, but i have encountered many of them. the drainage aspect cannot be stressed enough. you cannot rely on drains. they will clog up. a submersible pump is the only way to be safe, and that needs to be tested frequently. personally i hate that risk, so i would try to enclose it on 3 sides, while leaving the open side out of view. the vaults i've seen all have the slab you mention, and also have block walls around the perimeter. i've never seen one leave anything close to a 23" space between spa and walls except on the equipment side. you would cap it to avoid people stumbling in? standard vaults i've seen leave a 1-2" gap on the 3 sides, and are lowered in place by crane or with 4 men using 2 straps. the method for getting the straps in/out is place a 4-6 inch block under the edge of the spa by the equipment. if putting the straps under for extraction thread a strap underneath and see-saw it back and forth until you pull it to abouts 2/3rds of the way back. repeat with a 2nd strap at 1/3 back. then lift it out or attach to crane boom. its not easy but 4 strong backs can do it. i've been involved in the lowering in side, and my less than strong back did ok.