Just found this on the VS website.
Update on how to earn XP (Experience Points):
You earn XP for every bet you make on our Slots and Video Slots, except Progressive Jackpot games.
How much XP you earn per  £ 1 wagered is based on a specific game’s RTP (return to players), a game with 96% RTP gives you 1 XP, a game with 95% RTP contributes 125% to the XP giving you 1.25 XP, while a game with 97% RTP contributes 75% giving you 0.75 XP for every  £ 1 wagered.
Every 0.01% over 96% RTP of a specific game gives 0.25% less (max 75%) contribution to the XP. Every 0.01% under 96% RTP of a specific game gives 0.25% extra (max 75%) to the contribution of the XP. You can find all games’ RTPs on this page, please select “Theoretical” view of the RTP and not “Actual” view.
Examples:
Gonzo’s Quest has a 96% RTP so it contributes 100% to the XP. So, if you wager £ 20 on Gonzo’s Quest, this will give you 20 XP.
Starburst has a 96.1% RTP so it contributes 97.5% to the XP. So, if you wager £ 20 on Starburst, this will give you 19.5 XP.
Irish Eyes 2 has a 95% RTP so it contributes 125% to the XP. So, if you wager £ 20 on Irish Eyes 2, this will give you 25 XP.
No game can contribute less than 25% or more than 175% to earn 1 XP.
So basically this also means that to complete the  £ 500 wagering requirement to enter a Freeroll Battle, you would need to collect 500 XP. So from now on you can keep track how many XP you earn to know when you can enter a Freeroll Battle