The node which hosts the accounts will have full visibilities of the data on the account's sub-vault. You can view the accounts features as an isolation between different accounts, but since they all sit inside the node's vault. The node can see it all.
However, separate the private keys from the parent node is on the future roadmap of account.
For details documentation of account, please view: https://github.com/corda/accounts/blob/master/docs.md