Correctamundo!

Wrong! The Vasda Nevrada are living creatures so by infecting the Angel they are observing it therefore the Angel can not change back from stone so you run.

actually, the doctor who wikia describes the vashta nerada as microscopic swarm creatures. if the angels stopped moving because microscopic creatures were looking at them, they would never move

But this swarm also has sentience capable of thought so its not your average microscopic lifeforms.

but stil microscopic, the angels don't choose when to turn to stone based on the species inteligence

Where does it state that size was a requirement to trigger the defence mechanism?

nowhere, but it is the most logical scenario. if their defence mechanism activated for every thing that looked at them, they would never be able to move because of all the bugs and microorganisms around. even on your skin there are over 1 trillion microroganisms

But Microorganism's usually don't have eyes.
and a bugs sight might be very limited. so perhaps if the brain can't process it's existance visually then the angels are free to move.
So if an angel where on the moon and you where looking at the moon, the angel would still be "free" because it's so small your brain can't process and proove it's existance.

So it must activate in the presence of sentient lifeforms which the VN are or things that can harm it because it doesn't get stoned for a bug but I'm sure it would for a tiger.

Pretty much this, the precise term used it "quantum locked" and although i am not a physicist nor have i taken any physics ******* , i, like most people on the internet, know the basic Schrodinger's Cat quantum physics. The jist of all of this being that an angles freezing would be based more on being perceived than seen, i.e sentient things like humans, aliens, and yes, thinking swarms, would perceivedthe angles and thus cause them to freeze.

this just made me think of the episode where the angels were on the ship regaining their energy. and they went into their quantum lock because they thought any could see them even though she had here eyes closed

Yea, In all honesty I think that part was total bs. The angles don't have a choice about it, its an immutable law of physics due to the say they are constructed, on second thought its fiction and i honestly have very little idea what I'm talking about.

amy had her eyes closed, darn it

Take a thumb, because that is one of my favorite gifs of all time and i don't see it nearly enough.