my opinion is that the fact wallpapers are offered for free is immaterial, but the fact they are including your vector in a product for redistribution is usually not allowed by most standard licences, and at least an extended licence is required
should be checked against actual terms from the agency they used to buy your file; then contact both agency and buyer, offering to buy an extended licence as a way to resolve dispute - you cannot realistically expect more than that
maybe they have a special exoneration from God
