When the sky is obscured and cloud details cannot be assessed but information on vertical visibility is available, the cloud group should be replaced by a five-character group i.e. VVhshshs, the first two characters being VV followed by the vertical visibility in units of 30m or 100ft.

First two characters being VV followed by the vertical visibility in units of 30 m or 100ft, as for cloud base.
For Example: VV003 (vertical visibility 300ft) When the sky is obscured and vertical visibility cannot be assessed, the group will read VV///.

NSC-No Significant Clouds
In some regions cloud information is limited to cloud of operational significance. This is considered to be cloud below 1500 m (5000ft) or the highest minimum sector altitude (HMSA), whichever is the greater, and cumulonimbus or towering cumulus, whenever forecast. 

Therefore, when no cumulonimbus, no towering cumulus, or clouds below 5000ft or below the highest minimum sector altitude, whichever is the greater, are forecast and CAVOK is not appropriate, NSC (Nil Significant Cloud) is used.
For Example: When the prevailing visibility is expected to be 8 km and altocumulus and cirrus clouds are forecast above 10000ft, the cloud group would be replaced by NSC. If the prevailing visibility is expected to be 10 km or more with the same cloud conditions, then CAVOK should be used.