Virtual Service Account represents the Windows service. It is a built-in least-privilege account that has network log on permissions. This account is recommended if you don't have a domain user account available or if you want to avoid any service disruptions that might occur as a result of password expiration policies.
Network Service is a built-in least-privilege account that has network log on permissions.
If you have a Windows domain user account that has the minimum permissions required for report server operations, you should use it. A domain user account is recommended because it isolates the Report Server service from other applications.
For more details, you can refer to this link: Choosing an Account.
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