if using AP flour, it is okay to add vital wheat gluten?

This is my go-to baguette recipe, and I agree with AJ. You don’t need it.

Yes, you certainly can, but it’s not necessary. The recipe was tested with all purpose flour, so that should work fine by itself. Normally you use vital gluten when using a lower protein flour, such as rye flour, or when you have a fair number of heavier ingredients like seeds or nuts in the dough. I would only use it in a bread like this if the recipe called for bread flour, and all I had on hand was all-purpose flour.;o)