eric136,
Caveat...I'm not a vinyl PB.

The vermiculite provides a subsurface that will not settle if properly installed. It also provides a barrier for rodents that might dig. I've seen gofhers at a depth of 5-6 feet. The finish dimensions are important for a good, long lasting liner fit. Vermiculite allows this.
Some foam insulations don't hold up well underground.
Just a few points to ponder...Good luck.

Understand that most of the pressure is on the walls of the pool not on the floor and I know from the pools I have seen that have put foam on the floor it tends to shift overtime and become a mess. For foam board I have not seen it installed and would worry about the liner in the slope areas and creating footprints as the adhesive breaks down. A sand portland cement mix will give you the best floor and its working time will be a little longer than Vermiculite. I only use vermiculite to patch a floor for a liner replacement..