Hi,
Your understanding is correct. Single SnapVault license key unlocks both Primary and Secondary SnapVault.
Also Note: Beginning with ONTAP 9.1, you can use a SnapMirror license to enable SnapVault in SnapManager or SnapDrive. Those products did not allow you to use a SnapMirror license to enable SnapVault in ONTAP 9.0.
https://library.netapp.com/ecmdocs/ECMLP2673318/html/GUID-73A89D64-903C-4C65-A808-132DB3C205A4.html