You might want to remove the sand, put down landscape fabric, and then reinstall the sand. Weed-X AT fabric claims to stop ants and termites. It is treated with something, so you might want to check that the treatment chemical would not adversely affect the liner.
Even though the liner would not be in direct contact with the fabric, the chemical might migrate through the sand and get to the liner. Instead of reinstalling the sand after installing the landscaping fabric, you might want to use a vermiculite base, which would be a more solid barrier to intrusion.
Weed-X®AT is a treated material which provides the added benefit of ensuring that ants and termites won’t damage the material. This is especially important in areas of the country where fire ants are a problem.
http://dalenproducts.com/faq/landscape- ... pe-fabric/
http://dalenproducts.com/products/lands ... t-termite/
[edit]The landscaping fabric treatment chemical is Bifenthrin, which I think is safe to use with near vinyl, but you might want to check with the liner manufacturer just to be sure.
The product website does say "contains Bifenthrin: Not for use in edible garden, pond or pool applications!", but I think that that's only for direct contact with the water. You could contact the manufacturer to be sure.[end edit]
Instead of, or in addition to, the landscaping fabric, you could use an EPDM rubber pond liner below the sand. Because the liner is not permeable, I think that you would want to avoid creating a situation where the pond liner could potentially trap water against the steel walls of the pool as that might increase the risk of wall corrosion.
