I disagree on this one. Since what exactly happened between his fall into the Netherworld from DMC3 and his actual defeat by the hand of Mundus is not known, you can, in fact, make a game out of it. Maybe most of us just imagine that facing Mundus right away after his fall is what happened but they could make it so that before getting to Mundus himself, Vergil had to get through the whole Hell (Mundus is the Demon King after all, makes sense you gotta get deep into the Demon World to reach him) itself. So you could make a proper game, set in post-Sparda rebellion Hell (come on, how cool would that be... we've only ever had little segments in Hell from the games so far), with legit bosses and wicked as **** enemy and level design.