I was wondering about this question for a while but never bother asking.. Maybe this is the right time and thread to ask 
what's the benefit of putting our btc on vault? does the amount slightly increases by time? Or... its just safer that way? and what's the difference with the basic "normal" wallet?
what's the benefit of putting our btc on vault? does the amount slightly increases by time? Or... its just safer that way? and what's the difference with the basic "normal" wallet?
If I'm not mistaken, Xapo vault is a way to keep your bitcoins on a cold storage, which is more secure than normal wallet because they are stored offline.
If you use a vault, you have to pay an annual fee that works as an insurance for keeping your bitcoins safe.
You are correct that moving your funds to your Vault will put them in deep cold storage on servers that have never been connected to a network. So there is a miniscule risk that your funds in the Vault can be hacked. The timed withdrawal of 24-48hrs happens because different individuals in different continents need to manually approve the withdrawals (in which case our servers that again, are not connected to any network, will confirm the withdrawal). So besides a timed withdrawal that gives you, the owner of the btc, to stop the withdrawal if there is a problem, the funds are also being moved from safe locations. There is also insurance on the bitcoins stored in the Vault.
However, one important thing to clarify is that there is NO ANNUAL FEE to use our Vaults or insurance. It has been free since January:
https://blog.xapo.com/xapo-launches-suite-of-new-security-features-announces-xapo-vault-now-free-of-charge/