Except it's not a burst spell, because Vlad isn't a burst mage (unless you count a 6-8 second rotation as a "burst"), but the damage from AP (i.e. the ap ratio) is amplified up to 100% at 4 stacks, giving him an effective .9 ratio on the spell - that's HUGE, and you get to do it twice (even 4 times if you build ZHG, have a Zil/kayle, or something like that).
I've played Vlad quite a bit and memorized quite a few mechanics, also have looked my stuff up.
Originally Posted by LoL Wiki
Upon cast Vladimir also gains a stacking 'Empowered' buff for 10 seconds that increases the effectiveness of healing and regeneration upon himself, and increases the base damage and health cost of subsequent Tides of Blood casts.
I'm too used to only needing to rotate once, thus I do refer to it as burst, even if it's more DPS burst.