I have a sand bottom in my pool, and sand coves with the vapor barrier in an L shape to hold the sand back, and i can attest to hrdennis' statement, if done right, sand bottoms are excellent and also the cheapest way to go.
I took my time making sure it was nice and flat (although I did dip it 8" deeper to the middle which is REALLY hard to do with rigid foam), and i spent a lot of time compacting everything well.
It's a bit more involved to get the liner in without causing dents in the sand, but it's not impossible at all