The Vicuna is a very fast animal and the top speed that a Vicuna can run at is 29 miles per hour which is pretty darn fast.
That's faster than most other animals can run and a lot faster than any human can run.
So if a Vicuna were to run after you the only way a human would be able to outrun a Vicuna is in a vehicle going 40 MPH or above in speed.
Otherwise if you were to just run away from the Vicuna you would not be able to run fast enough to outpace or outrun the Vicuna.
However it's unlikely that a Vicuna would want to attack a human as long as the human doesn't try to harm the Vicuna you should be alright around them.