We are currently in the planning stages of an 18X34X48, oval Radiant pool, semi or fully in ground. The installer said he puts down a vermiculite base. Is this any better than sand? Should sand go over vermiculite or padding over vermiculite? The installer quoted $7000 for excavation (no dirt removal), plumbing, cementing around bottom of frame, vermiculite, and backfill? Electric is not included. About $1000 more for a deep end. Any thoughts on this? Thanks for any suggestions in advanced.