Vault Service account is a special account and member of the Administrators group, therefore it has access on every Enterprise Vault computer in the Enterprise Vault Site.
If you want to change the vault service account do it trough the Valut administration console as mentioned here:
1.Open Administration Console.
2. In the Administration Console, open Directory Properties and click the Service Account tab.
3. Change the details to point to the new account.
4. On each Enterprise Vault server in the local site, restart the appropriate Enterprise Vault services.
Login with the new credentials.