Keep in mind, I don't play Vlad, so everything I say is purely theoretical, but I do like theory-crafting a lot.
Deathfire is the best AP damage mastery in late game anyway, better than Thunderlord is most situations, and people under-estimate it a lot, thinking it's only good on Jhin, etc. But it's especially good in prolonged fights, and with AoE, because it can hit multiple enemies and it doesn't go on CD after a proc, so it's good for DPS champs. I'd say it is a pretty good fit for Vlad, even on Live, not only on PBE.
But the main question is: do you actually want damage or not? And an important side-question is: what else do you want to get from your Masteries. Going 18-12-0 for Dangerous Game might be interesting, but then you give up the Summoners CDR, this kind of trade-offs.
Overall, I'm pretty sure that going 18 into Resolve is now a bad idea. Grasp is reduced since you're ranged, you can't proc Colossus, and Bond of Stone is more a supportive keystone. Going 12 into Resolve is probably the best secondary tree, though, for Runic Armor and Summoners CDR.
Then you have to choose between utility in Cunning (Stormraider, 45% CDR, Dangerous Game, ...) or Damage in Ferocity (Deathfire, Sorcery, Oppressor, the %Pen, ...).
I'd say it's up to you and your playstyle and that one isn't really better than the other.