When they're both draftable, Vanessa saves a ton earlygame, but leaves the player without a decent bosskiller for the rest of the game, while whoever has Franz should not be too far behind Vanessa early on, and becomes a bosskiller himself. Like Integ might have Vanessa, Tana, and Syrene, but if his Eirika gets strength screwed, who is going to kill Callaech?
But without Franz in the draft, there is nobody to equal Integ's early game, and he's gone kick our asses unless he suddenly can't remember how to play FE8.