I have Firefox 57.7.2esr, but when I try to download Firefox VPN I get a message saying I only have 57.0. Why?

You have 5'''2'''.7.2esr. (fifty-two.seven.two) Firefox 5'''3''' (fifty-three) and later does not run on WinXP/Vista. What VPN are you referring to as there is no Firefox VPN product or service from Mozilla. If you mean one of the VPN related extensions being hosted at https://addons.mozilla.org/firefox/tag/vpn then many of them are for Firefox 57.0 and newer with current version. On the extension page scroll down to Version History > See complete version history and scroll down versions till you get one that work with 52.0 in rang of Firefox versions it works with.