Think I'm going to trust this guide more than you:
http://www.icy-veins.com/wow/vengean...-stat-priority
Crit is the worst stat for survival, aside from the immensely small amount of parry it gives you also need to crit on soul cleave to really gain anything out of it. Haste is great to increase the up-time of your demon spikes, which also happens to give you parry...
I've tanked all 8 mythic dungeons as vengeance without any problems, went in there with about 815 or some such item level. I'd highly recommend that you make use of your sigil of fear and imprisonment spell, some packs (especially two certain wrathguards up some stairs) deal a lot of damage and until you get used to it you really should make use of your entire toolkit to survive.
The second last boss for example moves really slowly at all times, the more damage he deals to you the more often he will use his Brutal Haymaker. If the damage becomes too much don't hesitate to use your infernal strike to create distance, be it on a boss or trash pack it can give your healer several seconds to heal you back up as well as refresh your demon spikes.