Vlad was not designed as a sustain mage.
His Q only healed enough to offset his W, E, and R costs.
W and R being % was too much, so they removed the cost from his ult. W is rarely used, so people got the idea that he was a drain tank due to his E costing so little and even amplifying healing.
When people think of Vlad, they think of 'drain tank' because of WotA, not Vlad's base kit. Anyone would realize that if they ever played him without WotA.
Now that WotA's healing is essentially included in his kit, the nail's in the coffin for that horrible design direction.
Yet, Vlad is still damage first and foremost. You can't win solely through sustain or Aatrox and Warwick would be considered much better than they are. They don't do damage like Vlad does, which is why Vlad continues to function despite being gutted repeatedly.
Damage is the key element of Vlad, not sustain, as you can tell with the tank build being nerfed; it hardly functions now. Why? Because sustain (the only other thing Vlad has) can't carry a team. Damage can.
Abyssal grants more MR, and replacing abyssal with SV only grants you ~400 (losing 10% HP for way more than 10% damage increase is worth) more HP. Healing is around the same for both builds, but full AP does WAY more damage.