I think basing the Angels on the episode with the crack (The Time of Angels) is very faulty. Basing it on the established rules of their first episode they appeared in (Blink) seems to make more sense.
I mean, think; they feed off potential energy (Time of Angels) rather than time energy (Blink). Well, potential energy's everywhere, so it doesn't seem to make sense that they need to hunt - simply standing near a planet would do.
In Time of Angels they say "the image of an angel, itself becomes an angel", which is ludicrous. That kind of thing would mean anyone that ever looks at one (not just Amy) should have been infected. Standing near water or a mirror should also create one. The amount of infinite regression is staggering to the point of lunacy.
In other words, I'd just ignore that whole episode for Angel motivations and abilities.
By doing so, we can easily work out what "being observed" means. It can't be the strict quantum mechanical "observed", because that refers to when any particle interacts with another particle - regardless of whether there's a consciousness behind it.
"Being observed" in this case seems to refer to conscious awareness of location. Simply remembering where they are doesn't count. It must be straight up knowledge.
At the same time, Angels must take care not to observe each other (another losing point for Time of Angels, as that many Angels should be slowing each other down through observing each other). Since we can rest assured that real-time knowledge of location counts as observation, this means that alternative forms of obversation also apply to Angels observing each other.
This means that Angels either lack sensation of vibration (else two Angels simply standing near each other would be unable to move), or can move without causing vibration. I'd more likely side with the later, which actually makes some sense, since the Angels have wings and thus perhaps "float" towards their prey.
This can be explained by the fact that, being creatures of quantum nature, they don't fully exist when not observed, and thus don't have feet to step with - only assuming the image of an angel when observed... an angel already on the ground.
Oddly enough, this would suggest that when characters "hear" an Angel, it is the Angel wishing to be heard.
So I don't believe Toph could stop Angels, as she would likely not sense anything - being that they aren't actually touching the ground. Should one become locked, however, while touching the ground, the only way to know if Toph could stop it from becoming unlocked again depends on what the Angels are made of. If the Angels are made of the "earth" element, she could stop them, as she would then sense their full form, and is observing THEM. If they are not, Toph would simply detect that the earth they are standing on is no longer being stood on. As we have said, memory of where an Angel is cannot be used to lock them, and so detecting that "they are no longer there" does not count as observing.
An odd question arises now - time delay. What is the difference between seeing and remembering what you saw? It takes light a certain amount of time to reach our eyes. Theoretically, you are only remembering what has happened. But since we know that remembering where they are does not count as observing, we must assume that there is an observation window of time.
This would allow Angels to move outdoors. Without such a window of time, Angels could never traverse the surface of a planet, as sentient beings (an established fact in Doctor Who as existing) on other worlds that look up into the sky would count as "observing" the Angels (even if not aware of it).
Thus, I believe a blind person would be helpless against an Angel just as the plants around him or her are held equally helpless.
(posted from my response on G+, thread by Doctor Who)