I know that its allies will only be boosted in double and triple battles, but will it work for Victini in single battles? Bulbapedia says nothing about it.
>You and your team mates receive an accuracy increase. Accuracy for the Pokémon and Allys is raised by 10%
Clarification: On the Smogon page for Victini in X/Y (Singles) it says this; >Bolt Strike hits the bulky Water-types that resist Fire, 2HKOing 248/244 Slowbro, and has 93% accuracy with Victory Star.
This implies it works fine in singles, or else Smogon wouldn't be doing calculations with Victory Star factored in.
Serebii