Should I use DFG before or after Vlad ulti?

It kinda depends if you're trying to do single target damage or AoE damage. I'd recommend DFG then Ult for single target, and Ult then DFG for AoE. You'll have to be more careful in a teamfight, and you want your ult as early as possible in it. Thus, DFG + Ult for single target. Ult + DFG for teamfights.