Making a spell miss it's intended target

I realise there are various spells that can cause an indirect attack to go astray. eg a Gust of Wind or Wall of Stone could stop Vilano's Sling from reaching it's intended victim.

But what about direct attacks, where the spell directly targets the victim?
If my grog is fighting the diabolist's grog, and the diabolist targets my grog with a.nasty hex, can my mage redirect the hex, to say, the diabolist's grog?

Modifying a spell effect is the realm of Muto Vim. In order to affect another spellcaster's spell, if you are not cooperating such as in your example, you would need to be able to fast-cast it, and you would need to beat the opponent's penetration total with your own to hi-jack the effect. Redirecting a spell effect towards a different target that you choose is the second guideline - a significant change. There is a catch, however - because Muto Vim effect can only work on hermetic magic. So you couldn't use that approach with an infernal effect. For non-hermetic effect, Perdo Vim is an alternative approach - simply dispel what's incoming. It requires control of several effect types, however.