Tricky one - I can see both arguements for this one.
A) Dispelling 'Throne of Vines' make 'Flesh to Stone' revert to its standard spell description, so +2 Toughness.
B) By casting 'Throne of Vines' it replaces the description of 'Flesh to Stone' to be +4 Toughness, to remove this bonus, you need to dispel 'Flesh to Stone' not 'Throne of Vines'
This does need a
FAQ, but can see
GW saying that the Players who's turn it is decides.....
Personally I'd go with B, as you did manage to get the combo off - forces the opponent to have to dispel the cause not the augmentor.
Going to have to have a good look at Lore of Life - some very good spells there.