Well, you need the Rabadon's, Zhonya's, Void Staff, and probably the Sorcerer's Shoes for every game.
You need some form of mana, and as a stacking AP cooldown limited champion you almost always want 40% CDR. That leaves Seraph's as really the only item to switch out.
And since Seraph's was just buffed, it's about equal to RoA (probably a bit better tbh). Veigar is an assassin, so getting the Rylai's slow isn't that good; most targets either will die in less than a second, negating any use of the slow, or kill you right after burst/ you walk away, making the slow rather meh as only your Q can apply it in a useful fashion (since you need EW to hit W most of the time, and they'll be stunned for the entire slow duration).