I myself not a fan of VIP system based on deposit because I prefer VIP system based on wagering because I'm small player only who wont be able to make big total deposit in short period of time.
Same on my side, I prefer to gamble when I feel like it. So actually this system is not suitable for me, luckily WE88 has other advantages from the promotions & bonuses they provide so that I still play on this platform.
I don't think they'll let customers to deposit and withdraw that easily. There should be some terms, like wagering deposit to claim it as a deposit meant for increasing VIP level because by simply making a deposit and withdrawal, their VIP system will be abused.
Turnover is indeed necessary, as @ryzaadit said. BTW, in addition to preventing abuse of VIP system upgrades, turnover is actually enforced for AML policy. Don't know if WE88 implements it for that purpose too or not.