Its been couple of days now and I honestly cannot complain about the vengeance spec. The sigils work like a charm in Mythic dungeons, the whole rotation can be subjective, but if it works for you, them thats fine. I compared my DK tanking (with legendary ring) to DH tanking and I really don't see the any weakness with the DH (known for being squishy for some odd reason) The 700ilevel gear from vendors mainly focuses on DPS spec rather than tank, perhaps critical strike for example is pretty much mediocre for tanking as it is the lowest from the stat priority, so I focused on mainly Haste > Mastery > Versatility.
I tanked HFC Mythic, HFC HC and also Mythic Dungeons with randoms, and I cannot complain, I had few deaths here or there, but it was mainly the healers faulty (as tanks more relay on them now) or someone from the group pulled more mobs with barrage (hunter).
Personal opinion, but I actually think DH tanking has a better edge than any other tanking spec, due to mobility.
But hey, this is just my experience, as I played it only couple for couple of days. We really don't need more talents at the moment.