The text of the [mwcard=DNJ12]Vine Tree[/mwcard] reads: "During the Deployment Phase, you may pay 1 mana to place 1 additional Vine marker with your Spreading Vines ability."
This means that when the Druid uses her Spreading Vines ability she may place 2 markers instead of 1 if she pays 1 mana to do so. There is no separate casting for the extra Vine Marker, it is not cast by the tree, and you may not use the tree's mana to pay for it.
