I wouldn't put any FVS in a druid caster build.
Anything but pure will seriously compromise your Earthquake DC if not geared/built properly.
I personally wouldn't risk losing the best CC spell just for some direct damage.
I have very little need for direct damage as a druid caster anyways. It is all easily covered already.
Besides the best druid direct damage is Creeping Cold and Greater Creeping cold, and no FvS enhancements will boost your cold damage.