The fog only grant obscured to object in arena. Zones are not object, so they are not obscured. So you can still use Vine Marker to target their zone (aka summoning a Creature).
So dont panic.
From Druid v Necromancer rulebook
Obscured:
"This object is difficult to see. It cannot be targeted from more than one zone away (this is for all purposes - ranged attack, spells, abilities, etc.) "
Vine markers:
"As the druid, or a familiar or tree spawnpoint she controls, casts a vine spell, you can destroy a
target vine marker she controls as an additional cost to cast the spell."
Rolling fog gives the vine markers obscured so she cannot activate her ability if she cannot target it. She can still place them, as per spreading vines, as long as they are adjacent because placement does not require LoS.