Can I take the ETH that I send to my USDT and XHT wallets or are they lost in the ether?
When you deal with ERC20 based tokens you better set up an Ethereum wallet with the exact same seed and that way you could see exactly how much eth you got on Vault.
You have to think of those ETH sent to the xht and USDT wallets as a fuel pretty much for transactions on Ethereum blockchain. It does not make much sense to transfer those in practical scenarios. Just check the balance and make sure you got enough for transacting ERC20 based tokens.
Tip: You can check the balance of your master (withdrawal) address to see how much ETH you got on that address on any Ethereum block explorer like https://etherscan.io