The VSG is just some ablative wounds that come back 1/3 of the time and require your opponent to be able to glance AV12.
If your opponent can reliably put 3 hits that glance AV12 first, he's negated your VSG and the remainder of his guns shoot you normally. In subsequent turns he only needs one such shot. 4++ is 4++.
Now if your opponent was planning on putting out lots of say S7 AP2 fire, the shield will eat 9 hits. If he can't break the shield the rest of his guns are useless that turn.
Honestly the VSG probably works best when maxed out.
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