This might be the case with pub Inferno games (I only play Inferno with friends), but I definitely don't have this problem in Nightmare or Hell public.
I don't want to say this but you keep bringing up Hell and Nightmare...
Nobody is having problem with Hell and Nightmare.
The problem lies in Inferno. This is the end game. Not everyone is interested in leveling every class to 60 and then stop. They want to play in the highest difficulty and farm the best gears.
It's either too hard for some, or nothing left to do to those people who've finished it. Try joining a public Act 3 and Act 4 Inferno game and tell me you don't see this problem.
Limited number of random events and dungeons, random dungeons that are not really that random at all, ridiculously hard to find gears that are usable or sellable, lack of interesting items. There's really nothing much to keep people playing.
It might not be a problem for you but it is to many, many players.
Vault has no invincibility frames, unlike dashing strike. You're not supposed to use it to dodge attacks. You're supposed to use it to create distance. However, you can use dashing strike to dash in place to avoid freeze orbs, hits, etc etc. It doesn't suffer from the gear-check philosophy.
Once a mob starts its melee animation you'll be hit unless you use some kind of invincible skill or stun it. It doesn't matter if you're a screen away. It's been confirmed by the devs. That's terrible combat mechanics. Unacceptable in any action game.
There are also input and server lag. Even if you use SS you will still often be hit unless you have a super fast connection. Many people have the same experience.
You might not notice it because mobs can't 1 shot you until Inferno Act 2 and beyond.