The most common difference between a llama and alpaca and a vicuna is there ears.
Llama's will have have straighter shaped ears whereas vicunas and alpacas ears will be more curved in shape.
A llamas face is also straighter than an a Vicunas or Alpacas face which is noticed from the side when looking at them.
Llamas, Alpacas and Vicunas are cousins to each other but are different even though from a distance they can look similar.
Another way of telling them apart is Llamas have a courser outer coat on them than the Vicuna or Alpacas do.