Why Chain Gang rune? You have a no cost vault (more on that later) so you can use caltrops quite heavily plus you put the immobilization rune on it. That is plenty of slow so I don't see why you need to hit four targets with entangling shot. What you do need is damage. Entangling shot does not do comparable damage to most of the hatred generators like bola shot or Hungering Arrow. If you wanted to keep a majority of this build intact, you would want Justice is Served as your rune to generate extra hatred instead.
Lack of AoE / general dis-synergy of abilities
You have no reliable AoE in this build. Entangling shot hits four targets, but not for very much damage. Rain of Vengeance is good.... on a 30 second cooldown. Impale is not made to be for AoE and Overpenetration helps but it isn't a substitute for real AoE. You put Valley of Death on Mark of Death to increase the damage taken in an AoE radius, but again, you have nothing in the build that takes advantage of that. Switching out Entangling Shot for Bola Shot or switching out rain of vengeance for multi shot should help. Switching Impale's rune to something that actually augments its single target damage (what it was made for) and getting better AoE elsewhere is the way to go. Otherwise you might as well switch the rune on Marked for Death and make this a single target build.
Acrobatics on Vault
Right now Acrobatics is overkill with this build. You have entangling shot slowing 4 people so you won't need to be spamming caltrops. Vault does not cost that much discipline and having it available on demand is infinitely more beneficial than having it cost no resources but only being usable once every 15 seconds. Heck, I have to use it more often than that to keep away from the Skeleton King in the beta. If you're worried about discipline then at least go with Tumble so it costs less but you can use it more than once. Switching out Steady Aim with Perfectionist for a 20% discipline cost reduction would also help. It'll be harder than you think to keep people 20 yards away from you. Especially if you can only vault once every 15 seconds. Unless you want to run for 5 seconds to get off 2 attacks and start running again.
Right now you're lacking synergy in this build. Cull the Weak in combination with ES and Caltrops is about the only thing I see that actually goes together. You have Archery but no crit-based skills or an emphasis on damage above all es. Most builds are either exceptional at one thing or well-balanced, but yours is neither right now. You aren't going to be the best dps single target (impale has much better runes for that) but you also can't AoE at all. You could change a few spells around like I stated before to bring up your AoE damage without lowering your single target at all. In fact it would allow you to switch out a sub-optimal rune on Impale and get better damage with it all. http://us.battle.net/d3/en/calculator/demon-hunter#bZQdYj!fce!aZbcZZ
Here is a variant on your build that pumps up your AoE and single target damage. It also favors regenerating/keeping discipline because Valley of Death is a fairly small AoE that you may have to move a lot along with laying new traps and vaulting. I kept the ES rune because I liked the idea of Cull the Weak in the build and I wanted to keep that focus on slowing in it. Also, because I switched Vault to cost discipline again, you aren't going to be able to spam caltrops like you could before. Because I fixed the damage problem with other things, I felt justified in keeping ES and keeping Chain Gang on it. Now with multi-shot you can take advantage of what I presumed you wanted to do, which is kite a lot of enemies to a caltrops to freeze them in place, laying down a Valley of Death, and laying waste to them. Any smaller groups / strong groups you can keep slowed with Entangling Shot both keeping them at a distance and keeping the extra damage buff from Cull the Weak active. Then you can pick them apart with Impale or Multishot depending on how many of them there are.