Will a copy of a flashed-back Increasing Vengeance still make two copies?
No, it will not make 2 copies. See the ruling on Increasing Vengeance. 


  When Increasing Vengeance resolves, it creates one or two copies of a spell. You control each of the copies. Those copies are created on the stack, so they're not "cast." Abilities that trigger when a player casts a spell won't trigger. The copies will then resolve like normal spells, after players get a chance to cast spells and activate abilities.


The copies were not cast from the graveyard, so the replacement effect doesn't apply.