AP Sion is far from tanky, health alone is one of the worst tank stats and that's all Sion gets, Vlad at least has a pool and while not comparable burst, at least comparable damage.
With about 7000 gold spent, Sion can do around 540 single target damage and 540 aoe damage. Vlad can do 410 single target damage and about 550 AoE damage assuming he maxed pool first but discluding its damage. Vlad's spells also have half the cooldown of Sion's and he doesn't lose his main source of damage by being looked at too harshly.
TLDR, I have no idea why you'd pick Sion instead of just Vlad, his stun isn't that awesome.
He gets attack speed and Lifesteal, how is that any good as AP? I'd see the argument that you can have a high level on Enrage, making you do at least some damage with your autos but that means no maxing out other skills and by the time AP Sion is 18, the laughing at him just slows down to a mournful pitying.