Correct. 20xrp is required to activate your account on Ripple blockchain.
Is this forced by xrp system?
Yes. This has nothing to do with Vault and its the case for any Ripple wallet.
What happens to my 20xrp? Can I talk that later?
The 20xrp reserve is frozen and can not be withdrawn from what I know.
Does my wallet actually work without putting that amount?
Nope. You are required to put the reserve amount for each Wallet. So better you generate one wallet and generate multiple addresses under that wallet to avoid paying multiple reserve cost.