Jens
No, you don't need a Domain Admin Account.
You can make the VSA a "local admin" on the DC's by adding the Account to the "BUILTIN\Administrators" Group of your Domain. (Which is the "Administrators" Group for all your Domain Controllers).
But remember that granting BUILTIN\Administrators right gives the User the ability to promote itself to higher Permissions, so a "BUILTIN\Administrators" right is *almost* the same as a Domain Admin in the end....
Cheers
Michel