[Pocus]

even though water 9 is perhaps the best blessing for units with a good defence stat as Vestals, you can emphasize on some of (standard) Pythium strength, Air :

air 9 + air ward > a nice protection for a vestal based army with 2-3 archtheurgs casting wrathful skies.

fire 9 + mass flying, with a core of legionaires as bodyguards: flying vestals attacking with flamming weapon the rear (generally flyer manage to hit the rear while grounded hit the flanks of a large army). In essence they have first strike with a powerful flaming spear.