Basically, throne of vines may stop the miscast from happening. Cupped hands only works when a miscast actually happens.
There is no real conflict because you can't use cupped hands unless you fail the throne roll and thus, the miscast happens. When cupped hands comes into use, the throne of vines effect will have to have been resolved already.
So, you roll your throne of vines save. You roll a 1. The miscast happens. You then make your cupped hands roll to see if you're getting the miscast, or an opponent.
The hierarchy of what's happening is fine, and the throne of vines makes it clear, you have to roll to avoid the miscast.