Never had a problem with either DPs or with Vamps. They're easy. Dual wielders are child's play (particularly Briar Hearts?) Boy, you must be the most incredible fighter ever.
That or I actually play and analyse the game on higher difficulties and at lower levels than I have a business fighting at.
Dual wielders have a huge glaring weakness among melee fighters, making a sword and shield or a two handed enemy much more dangerous and difficult to deal with. Their attacks are typically slower and lock them in place, making it much more difficult for them to actually hit you. I'd much rather fight a dual wielding briarheart than really any of the other types.
Vampires are objectively the worst NPCs in the game. Specifically master vampires of an appropriate scale to your level. Dragon Priests are right up there with them. If you have no trouble with them then you should have no trouble with the game at all, as I can't think of a single NPC that comes close to their level of power, AI, and versatility. They have no exploitable weaknesses in their movesets, do tons of damage, heal themselves, have tons of hp, are fairly quick, and I could go on with the advantages they have over everything else. Every other NPC I can think of has exploitable weaknesses or simply doesn't have as many advantages, or both.
I've defeated a Master Vampire at level 1, and it was by far one of the most difficult fights I've ever had in the game. I did the quest where you kill that ghost guy that dual wields with his specially enchanted weapons in his cave full of draugr that spawn as the higher levels no matter what level you happen to be (at level one) and it was freaking hard. But it still wasn't as hard as a single Master Vampire.